I culture my own infusoria to feed to freshly hatched betta fry as I find that vinegar eels can sometimes be nearly the same size as 1 day old betta fry. To culture, I fill a quart size jar with established aquarium water and some vegetable matter. For the culture in this video I used grass and a piece of romaine lettuce as the vegetable matter since it was the most available to me. The culture is about two full weeks old.
I started the culture in this video from a culture I hadn't touched in over a year. That old culture was half evaporated, not fed for over a year, and there were still enough eels to start a fresh culture. And as you can see from the video, it is swarming with eels in every direction. So to me vinegar eels are the easiest and hardiest live foods to culture.
I wish I were able to replace brine shrimp with daphnia and culture it in large scale (in a large pond dedicated to daphnia), since they are so much easier to maintain. Because unlike brine shrimp, I do not have to setup a new culture every single day. However in my current situation I can only culture them in a 10 gallon which produces enough daphnia to feed the fry every few days. I purchased a daphnia moina online to start the culture in this video. They are cultured in a 10 gallon tank with clean aged water as well as siphoned water from an established planted tank for nutrients. Along with the daphnia moina are a couple daphnia pulex (much bigger than moina) that I hatched from eggs, scuds, and ramshorn snails. I feed the tank banana peels and zucchini which the scuds and snails break down.
These are my favorite foods for bettas. The larvae are free, easy to maintain, and come in a variety of sizes so bettas ranging from weeks old to years old can all eat them sufficiently as a meal. Mosquito larvae molt as they grow, as you can see in the photo below, they pass through four instars and then become pupae (which I also feed to bettas in the rare case that they get to that stage since I harvest the larvae everyday). That is why they are so versatile in feeding due to the distinguishable size differences.