Place a bucket outside (even in winter), toss in some leaves, maybe a tiny bit of dirt, grass.. and check it often. Daphnia will spawn in the water in winter, and you will get mosquito larvae quite a bit when it gets warmer.. the larvae are pretty safe to use, as they haven't had a chance to go out and eat on anything. Just rinse them off in a shrimp net and feed.
Live brine shrimp, blood worms are great- even fry of other fish work. Live food is great as long as you are in some control of where you get it.