A few good places to start are listed below, assuming you're looking for an actual person to teach / choreograph flag moves.  If you are looking for an instructional video, you can order one from one of the suppliers listed on my "Ordering Equipment" page.  They are expensive though. (Running between 40 and 50 dollars!)  If you need staff bad, here are some classifieds:

    Marching Links.com classifieds

Flag Basics

    Okay, here is an attempt at explaining a few things you absolutely need to know.  First off, let's talk about positions.

Right Shoulder - This is the most important position to master.  Almost everything stems from right shoulder, sometimes abbreviated RS.  Your left hand goes over the bottom stopper and your right hand grasps the flag at the tape. (The place right where the silk stops and it's bare pole.)  Now, no matter how short a you are, or how long the pole, your left hand is directly covering your belly-button, or the area right were your belt buckle would be.  The flag should be straight up and down and in the center of the body.  A good way to check this, is to make sure that the flag is right in front and in line with your nose.  When this is all done, the right hand should be just above or right at forehead level.

Left Shoulder - This is the same as RS, but the hands are switched.  However, this position is not very common.

Right / Left Slam - I'll describe this as Right Slam, abbreviated RSlam.  There are two ways to think about this.  Start at RS, and just bring the flag down to your right until the silk tip is pointed at the ground.  Like you're slamming it towards the ground. (Hence the name)  The other way is to put your left hand at the stopper and your right hand at the tape.  Hold the flag at a 45 degree angle up from the ground.  The flag should cross your body like a diagonal.  Your left hand should be in front of your left shoulder and covering over top the pole.  Your right hand should be a couple inches out from your hip and palm facing up to the sky.  LSlam is the same as RSlam, just switch your hands and the flag is on the other side of the body.

Right / Left / Front Present - Start out in RS and push your right arm straight forward until it is completely extended and parallel to the ground.  That is a front Present. (FP) Your left hand DOES NOT move from your belly button.  Make sure you're not hunching over or burying the stopper in your stomach.  Left Present is pretty much the same except the top hand moves to the left until it's MOSTLY extended.  Obviously one cannot extend their arm all the way without causing pain, or throwing off the angles in comparison with others.  Go until your elbow is in front of your chin.  Now, Right Present (RP) is the hardest to keep clean.  Try to keep the flag directly in front of the right shoulder.  If it doesn't cross in front of your shoulder, you're probably going too far.

Flat - This is the easiest position.  It's pretty much just the way it sounds.  Start at RS, and bring the flag down until both arms are extended down.  Keep your arms loose, and make sure the center of the bare part of the pole is in the center of your body.  That way your arms are out from your body at equal distances.  This can also be modified to eye level, where the flag is parallel and directly in front of your eyes.

Okay, now that we've got that covered, how about some spins?

    Despite the name, you don't actually "drop" the flag. (although my fellow flaggers might disagree :) Start at RS.  The flag moves counter-clockwise for this spin, since you are starting at RS. (This can also be spun clockwise if you start at LS, but that's a little more difficult!)  Okay, take your left hand off the stopper and let it hang at your side for now.  Move the flag counterclockwise until the flag is straight upside down.  As you bring the flag down, also bring your right hand down.  By the time you get your flag is upside down, your right hand should be in front of your belly button, right where your left hand used to be.  Now put your left hand flat out in front of you.  Your palm should be facing the ground.  Turn your hand 90 degrees clockwise until your thumb is pointing to the ground.  That may feel funny, but that's how you know you're doing it right. Okay, here's the hardest part.  Keep your hand palm facing to the left (thumb down) and grab the pole.  But you MUST grab below your right hand. (See crappy picture)  Both thumbs should be pointed away from you.  Now take your right hand off and let it hang by your side.  Keep the flag moving counterclockwise until it returns to being right side up.  Grasp it again with your right UNDER your left.  Guess what?  That's a drop spin.

Tips on spinning:

-Two words: Down, Up.  As you spin, say down, up, down, up...etc.  Trust me, it's a big help for beginners.
-Is your flag slowly rolling up?  Watch the way you're grabbing the flag...you are most likely twisting your wrists too much.
-Please, please, please make sure you are always grabbing under your top hand.  Stop yourself and check every now and again.  It's a difficult habit to break if you learn it wrong.
-The more you practice, the cleaner these will become.  Start off with a set of 50 spins and work up to two sets of 100...one set from RS, and if your are felling confident, 100 from LS.
-Make sure you're spinning at your waist.  If you spin too high up, you are making it much harder on yourself and wearing out your arms.

Peggy Spins (aka Double Fast)

    Peggy spins build off of drop spins, so this should not be practiced until you are comfortable with drop spins, otherwise it will be very confusing.  Peggy spins are also known as Double Fasts because it spins twice as fast as a drop spin.  I was taught to think of the first 4 counts as a unit. (Getting into Peggy spins is harder than continuing them!)

    I'm covering both styles here: Peggy and Double Fast Spins.  Double fast is a little easier, so if you're just starting out, I'd start with Double Fast.  Both versions start the same.  Count 1 is a drop spin.  Count 2 is where the variation comes in.

    Count 2 you keep spinning like a drop spin, but you go past vertical and stop when the flag is horizontal (flat).  You grasp the pole with your right, both palms up and fists right beside each other at the tape.  Count 3 you keep going and grasp it like a drop spin.  Count 4 you again go past vertical and grasp it with both palms up as the pole reaches flat.
 

Peggy:

    Count 2 of Peggy spins is what's called a "hitch."  To do this, you rotate the flag in your left until it is upright (like you would for a drop spin).  Put your right hand out like you were hitch-hiking.  Open your hand, so your fingers are straight and your thumb is sticking up.  Then, turn your palm so it is facing the ceiling.  Your hand looks like an "L" facing up to the sky.  Put the "L" up against the pole (under your left, of course) so that the pole is right up against your thumb pit.  That's count 2.  Count 3 you whip the flag around and grasp it with your left just like a drop spin...EXCEPT you break the cardinal rule and grab above your right.  Count 4 you go back to the hitch.

That's it for those spins!

The skinny end is called the nose. The fat end is the butt.

RIGHT HAND SPINS

1. Start with the nose facing your left. Hold Left hand on the end of the nose, palm facing down. Hold neck with Right hand, palm facing up. Slightly push nose down with Left hand (drop left hand to side) and and twist Right wrist so the rifle spins around your hand. You should totally let go with right hand and grap again as the rifle spins so that your hand ends up in the EXACT same place you started the spin.

When you spin, your wrist should move as though you were twisting a door handle. Make sure you do that. Put rifle down and make sure you're twisting your wrist right, side to side.

2. After you're comfortable with one spin, try to keep going without stopping.

3. DROP STOP Do your last spin and then while the rifle is in the air, switch hands so that left hand is held out to catch the nose when it comes down (about 9-10 inches in). As it hits your left hand, at the same time, grap top of the end of the butt with your right hand.

LEFT HAND SPINS

These are done the same way as right handed ones but your hands start the same way you end with right handed. Your left hand is under the nose about 9-10 inches in and right hand is on top of the butt.

1. Slightly push down with right hand and twist left hand the same way as you would a door handle.

2.Go about the left handed drop-stop the same way but end the same way you started with Right-handed drop spins. Left hand is on top of the end of the nose and Right is on the bottom of neck of the rifle.

TOSSES

1. Start the same way you would for a left-handed drop spin. Push down with Right hand, more than for a drop-spin but not by much. Twist your wrist until it your palm is facing left and thumb facing down. The butt of the rifle should be pointing straight up. This is the point where you release and toss the rifle. When you are beginning these tosses, you may have trouble getting it to go high enough to do a whole spin. Keep working at it and you'll build up the height. Make sure you push your arm straight up and not to the left. Catch with Left hand on the end of the top of the nose and Right hand on the bottom of the neck.

Big thanks to Sara for this great info!