You can sometimes get daphnia eggs from frozen daphnia. Buy a small pack of frozen daphnia from a pet shop and put them in a shallow container of water. Look for any daphnia that have round or figure 8 shaped black things in their body, these are the eggs. Put a few of those dead daphnia into a container of green water and see if any hatch. You can also let some of them dry out for a few weeks and then add the dry eggs to the green water.
If you have any freshwater lakes/ ponds nearby, you can take a fish net down there and collect your own daphnia in cool weather. Add them to a container of green water and let them breed. Move the babies into another container of green water and let them breed. Move some of their babies into another container of green water and let them breed up and use those ones for food. The first 2 cultures can be tipped out.
The idea of taking young and starting new cultures is to get clean brood stock that aren't carrying any parasites or parasite eggs/ larvae.
The following link has information on culturing live foods for baby fish. You need to start cultures now because most take a month or so to get going.
https://www.fishforums.net/threads/back-to-basics-when-breeding-fish.448304/
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Newly hatched brineshrimp are good foods for killifish and easy to hatch.
Many years ago we use to get buckets of apple cidar vinegar and leave them exposed to the air for a few months. We put a few slices of apple in the vinegar too. After a few months vinegar eels would appear in the containers and they were used as starter cultures.
White worms can sometimes be caught by cutting a potatoe in half and sticking it on the surface of the soil in a clean healthy garden. The worms are attracted to the starch in the potatoes and crawl up onto the underside. You check the potatoe each day and when you see little white worms crawling on the cut surface, you put it on a small container with some slightly damp peat moss or potting mix and let the worms crawl into that. They can be fed on dry powdered baby cereal but don't add too much cereal because it goes mouldy if not eaten within 24 hours.
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Check Ebay and other online vendors for live fish food, and try doing a Google search for live fish food cultures in your area.