Check out Amazon. I know I've seen the Scanmask ones on there before.
@kiwipaulie beneficial nematodes are microscopic worm-like organisms that parasitize a wide variety of soil-dwelling pests (root aphids, fungus gnat and even flea larvae). Their activity also benefits the soil by cycling nutrients.
These nematodes aren't easy to breed AFAIK, besides the fact that you need a microscope to determine how they're doing.