Buy Fruit Flies

I was creating a terrarium and I wanted to add some kind of animal life, I decided on small insects would be better. I wanted to have a balanced food chain (plants fruit flies "nematodes" Collembola). Where can I buy these or find them and, if possible, please tell me the ratio for each creature. thaaaaankyoooous

eBay is your answer to everything or try www.liveaquaria.com but they mainly fish srry