They're pretty difficult to find, even in a good book shop like Barnes and Noble. The best bet, if you want the actual book, is to buy them off of Amazon.com That's where I bought Justine. You should be able to find 120 Days of Sodom in Barnes and Noble, however, since that's where I found it. Buy the thicker version, if you find more than one--I have a feeling the thinner ones have been edited/censored or something--the novel itself is pretty lengthy, despite being unfinished!
I don't know where you live, but if you live in the UK, I heard Hesperus press was planning on releasing some of his works. I live in the U.S. so I'd probably have to wait a while for them to get imported here, if they ever do.