Another great place to get them are from friends who drink wine. I tell them once they have finished the bottle to rinse it real well (especially if it was a bottle I gave them). If they return the bottle(s) they will get a bottle or two back. People who constantly say "oh sorry, threw that one out by accident" can go buy their own wine.
I have LHBS that carries used bottles from a wineries tastings. They are $4.99 per case, usually really clean, sometimes without labels. The only thing I do is pull the bottles out (they are usually inverted in the case) and check that the openings aren't chipped.
The last case I bought was standing up in the case, and I remember thinking, that's nice, don't have to pull them all out and check them. I should have, they had dried wine in the bottom, and because of this thread I am thinking to mark them and keep them for a lower end kit I'm doing, and see if something funky happens after they have been refilled for a while.