There are many good produces out there.
We feed ours Attisons Betta Pro which you can get on ebay. They sometime have Hikari Betta Gold from the lfs if we run out. Once a week they are given live bloodworm, daphnia or brineshrimp or frozen if we don't have any.
It's also useful sometimes to give a small piece of cooked deshelled pea as this helps their digestion.