Ebay sometimes has good stuff. Amazon too. The classifieds on UM are worthwhile as well. Beyond that, Nuclear War Now! has very good prices ($5 on most CDs) if you're into that stuff. Century Media Distro has a huge clearance section that gets extremely cheap when they do half-off sales (every couple months). It's mostly a lot of obscure stuff ranging from rock, metalcore, hardcore, brutal death metal, black metal, and thrash (almost never any power metal). Worth looking through, though, and definitely contains some gems.