You need Surf and Fly.
1. Go to Viridian City and talk to the guy that said he just had his coffee.
2. Answer "No" to his question.
3. Fly to Cinnabar Island. In your items, put the item you want 100+ of in the 6th spot.
4. Surf up and down on the right side of the island, where it looks like your on half land, half water.
5. When you see Missingno or "M", run from it.
6. When you look in your items, you will see that your 6th item will have a 9 and a strange symbol next to it. That means you got over 100 of that item!

More codes to come!