If you want a pure strain of infusoria, try a visit to the local university. The biology departments often have one or more cultures of micro-organisms that can be cultured as fish food. Other options, which really are not the same as your request, is to use a green water culture or a nice big clump of java moss. Both contain microorganisms in large enough numbers to feed fry. For the java moss, put the clump in the fry tank and let them find their own food for a few days, then move up to larger foods. For green water, feed a few ounces per feeding of that really green water to the fry tank. The single celled algae are also a good fry food for very small fry.