You have a few options. I always chose to hatch my own baby brine shrimp until they were big enough for very finely crushed flakes. I've also known people to use cooked egg yolk. Mashed up real fine as these guys are TINY. OR you could use a liquid fry food, but these are rather messy and since its difficult enough as it is to clean a fry tank, especially when they free swim, I'd stick to the BBS.
hth.
Amber