A while back I came across a neat little website called Skywaypro that had some unlicensed games for sale. They mainly sell Neo Geo stuff, but they have some gbc games in the other consoles section. I went ahead and bought a couple of their cheaper gbc games (mostly unlicensed fighting games) and they shipped them reasonably quickly. The stuff listed under Other Consoles has some stuff that I think is decently priced, but their special items section is pretty expensive. I'll leave a link to their website if anyone is interested.
http://skywaypro.bravehost.com/otherconsoles.html