You can buy first bites which is a powdered food for fry or you can crush flakes up to a fine powder I would do the first bites myself though, they are really cheap.
It's good if the tank is cycled but guess you could still try if it's not, just do a lot of water changes and don't add gravel to the fry tank they bury themselves in it. Feed them by dipping the end of a tooth pick in the tank water and then into the powdered food the in to their tanks. I would feed them very small amounts every 2-3 hours if possible. Some say to start feeding a couple hrs after born some say not in the first 24 hrs. I never had any issue's feeding mine after the first couple hrs.