Other than "starter cultures" of tubifex, Aquatic Foods is the ONLY place I know of that sells them. With the shipping and handling charges, the minimum order is about US$22. But blackworms are essential enough to me that I buy them anyway.
One other option to consider is some kind of cooperative arrangement with a local pet shop (e.g., they get a larger order and split it with you ON THE DAY IT IS RECEIVED). Sadly, pet shops take care of their blackworms even less well than they care for other animals, so the worms deteriorate rapidly there.
