What is the best source for out-of-print RPGs?
If I am looking for a particular hard-copy of a game I usually go to eBay first. The pricing tends to be fair and they usually have what I am looking for. Amazon sometimes has them but not as often and not always at a good price, mostly because the sellers charge a lot. I also find myself buying from Noble Knight Games on occasion but they do not always have everything in stock that I came looking for; though their sales are pretty darn good.
As for pdfs of out-of-print games the first I check is Drivethrurpg. A lot of companies have started putting out pdfs of their old games and more seem to be doing it of late. They seem to be slowly coming to the realization there is a market for them.