As to finding stuff, Google Image Search is really good -- it has an option to search for 'line art' specifically. That gets you all sorts of black-and-white art on a certain subject. Excellent inspiration, and if you see something that's interesting you can see it in context and see what license it's published under.
Another good source of royalty-free images is Project Gutenberg. The advanced search interface can be used to search for image files as well!
Tattoo designs, spraypaint stencils and pumpkin carving patterns are quite good for offering vivid black-and-white art as well.