THR and Falfiles member kfranz has been known to sell a lot of used mags for good prices. Over the summer I bought 10 mags for $49 shipped. They arrived clean (no cosmoline), with various levels of finish, and almost no rust. 9 of them worked flawlessly right away, and the 10th one worked after I bent the feed lips a little. I would PM kfranz before ordering from someplace else, he has a very good rep on Falfiles and is nice to deal with.
Also, quite a few places sell mag spings, followers, and floor plates so if you get a bad mag you can usually get it working again.