Here are a few items that will come in handy when making bento.

Miniature Cookie Cutters
I bought my cookie cutters at a Michael's Craft Store. They had some in the baking section for fondant cakes and some in a section for cutting out wet clay shapes. They still work fine for food.

Bamboo Sushi Rolling Mat - Makisu
I have a mat for rolling sushi (called a makisu) but haven't attempted making any sushi yet.

Onigiri Mold
Onigiri molds come in many different shapes.

Shaped Plastic Sauce Bottles
These little bottles are used for holding sauces like soy sauce.

Wooden Skewers
For making meat skewers.

Paper Punches
I use these craft paper punches for punching shapes out of nori sheets.