Since everyone's already mentioned most of the best places to find them offhand, I'll just say that those gear motors of the style CDW linked should be checked out upon taking delivery. I have a couple of the exact same style that have a lot of gear lash, which makes them very noisy to say the least. That said, the other 8 out of the batch of 7 rpm versions were of much better quality, so ymmv.
Mine came from SurplusCenter.com btw. But they seem to be out of anything of reasonable quality > 14 rpm DC gear heads that isn't a super expensive Bison brand at the moment. They do have plenty of selection in the low rpm AC gear motors though.