Easy. What you'll want are simple brine shimp for your molly fry. You can buy thousands of eggs for $5.00 at any pet store. Create a brine solution in a small tank (betta size, 10 gallon or less) with plenty of aeration. Once the shrimp start hatching, do some 10% water changes until you eventually have fresh water w/aquarium salt.

Feed your bring shrimp a piece of cucumber every now and then, and scoop out the shrimp and feed them to your fry. They will LOVE them!