Raidforums and similar have them. You occasionally find them on reddit and small security blogs. I collect data breaches, and usually by the time you hear about one on HIBP it’s making the rounds on a variety of forums. Only on a few occasions has Troy actually gotten anything exclusive.

I don't know any off the top of my head (no, really.) but I'd bet searching DDG for "Hidden Wiki" and using tor hidden services would get you in the right direction, and probably put on a watch list or two.