You can try New Life Spectrum, either the Betta formula or the Small Fish formula will work great. They have garlic in them which entices Betta's to eat since it's so yummy for them! It's usually not too expensive, around 4 dollars for a decent sized container that's going to last you like ten million years lol I've had mine for 9 months now and it looks like I've barely dug into it and I've got about 33 fish to feed and I spilled some at one point >.< They're good because they're smaller than most foods as well.
Another you can try is Omega One Betta Buffet Pellets which are slightly bigger than NLS and costs roughly 3 dollars for a smaller container. My fish prefer NLS over Omega but both foods are very high quality and almost guaranteed your fish will like them. Sometimes though if they aren't feeling well, they won't eat as much so that's also to be expected.
Rembrandt II, Suiché
Leopards: 1:4 Hawkeye, Luna, Astrael, Dorian, Cullen
Gargoyle: Cloudjumper and Toothless