The two most recommended foods on this forum are New Life Spectrum (NLS) betta pellets and Omega One Betta Buffet pellets. People prefer pellets because they're easier to count out, and they don't make a mess in the tank.
Petco carries both brands (NLS and Omega One). Petsmart carries Omega One only. Or you can buy them online via petstores, or Amazon, etc.
When you look for a food, pick out one that has lots of whole fish. Avoid ones that contain a lot of grain products (wheat, corn, gluten, etc), as they can cause health problems such as constipation and bloating.