KEH is also a reputable source (http://www.keh.com), and has a decent selection of Leica screw-mounts. Their prices will be on the high side, but their grading is accurate and they're good to deal with. You can also use KEH prices as a guide to what the top of the price range should be when buying elsewhere.

http://www.donchatterton.com is another Leica source, but doesn't carry much in the way of screw-mounts - mostly Ms and Rs. His grading is a bit looser, but I've dealt with him a couple of times with success. Others, however, have reported mixed service.

Which raises the question - why not make the jump to an M? It would be more of an investment, but if RF is really to your liking, you're likely to get more years of service out of a more recent model.