I'll mention MissMiniatures just to give you some more options, although they are a bit on the short side, more like 25mm:
http://thescreamingalpha.com/?p=390MissMiniatures website seem to have disappeared though, so Ebay is probably your best bet here.
Having personal experience with both Mantic Ghouls and MissMiniatures, I'd recommend Mantic.
You could also use the ghouls from Castle Ravenloft. I've seen the minis being sold apart from the boardgame on Ebay.