I raised mine on vegetables, flakes or granules. Most livebearers eat a lot of veg or algae naturally, so I used boiled lettuce and spinach - available all the time for them to graze on and changed daily. Fry are ALWAYS hungry.
I live in Australia where there's a chain a bit like Wal-mart where they sell 'betta' food. It's totally unsuitable for bettas as the granules are far too small but it's high in protein and they grow very fast on it.
I also used crushed flakes. Put some flakes in a shot glass or medicine cup or something like that. Wet them and then add a few more mls of water. Use the tip of an eye dropper to break them up and then suck the resulting 'soup' in and out of the eye dropper very fast. You will end up with a heap of crushed bits of flake. Pour the liquid off because it will foul the water - only bacteria can eat pieces that tiny. Feed the rest to your fry.
You can also use live foods like brine shrimp or microworm, but these are more expensive and time consuming to produce. With livebearers, the fry are born so developed that you can start them on whatever the adults eat provided you crush it up small enough. I don't bother with expensive live food with livebearer fry because the difference in their growth and survival rates is too slight for it to be worth it. I save the feed money for buying really, really good food for the adults, which definitely thank you for feeding them well by being colourful and active.