Frozen bloodworms, brine shrimp, daphnia, or mysis shrimp can be found in many pet stores. As for live foods there are cultures of grindal worms, white worms, daphnia, or flightless fruitflies that you would have to order online or find someone that might have it locally. Some people also catch mosquito larvae or bloodworms from outside, but you can introduce something unwanted into the tank through that method.
Either collect them from vernal pools or raise them outdoors. For a hobbyist with one or two bettas, culturing is simple. A shallow pan with rainwater will get you plenty of mosquito larvae. Green water is great for culturing bloodworms, which are midge fly larvae.