I feed my bettas a staple diet of pelleted food -- Hikari Biogold in my case -- and supplement their diet with frozen foods, mine seem to favorbloodworms and mysis shrimp.
If you're looking frozen, whatever commercially available wormy thing will likely be devoured with gusto.
*edit: I've tried adult brine shrimp, and while I've heard of others having their bettas eat it, mine ignore it. My female betta often steals the flake I feed my other fish.
*editx2: I rewrote the ending to my first sentance, didn't look right to me.