Thanks for the help Vic. I may look into culturing them, but for the price and what (now) appears to be a reliable source I may just buy them when I need them. They aren't something I will be needing all the time.
Thedwightguy, Vic is correct, Tubifex and Blackworms are not the same thing.
http://www.skepticalaquarist.com/lumbriculus