alenppc,
The best place to find rare books is abebooks.com. Their selection puts amazon to shame, and you can find stuff that's quite rare and some even of historical significance. At one point, the SF writer Tim Powers sold his entire collection of Philip K. Dick novels inscribed to him by the author. However, at about $5000 per book it put most of it out of my price range...
There are also good deals to be found there by other dealers who simply deal in used books by volume and don't bother to check rarity, so it's always possible to get a good find.
