A bit more practical a question.
Not sure what the best source is, but some sources I use include DriveThruRPG, and RPGNow for pdfs, Noble Knight Games, ebay, and used bookstores.
Yes, used bookstores. Especially here in New York City, out-of-print RPGs often turn up at used bookstores. Rarely do you see the hard to find early stuff, but you never know, so it's usually worth your time to check in you're in the market for such things.
Besides, who doesn't love used bookstores?