Audiogon, Videogon, eBay and even the "for sale" section of this site.
Are you looking for a stereo receiver, one that has 5,6 or 7 amplified channels? If you are looking for a multi-channel receiver, what formats do you need it to process? DTS, DD, Pro Logic II?
Maybe, some of us can help in pointing you closer to what you are looking for.
If it's a 2-channel stereo receiver that you are looking for, don't overlook the vintage gear. Something old by Marantz or Sansui would be the ticket. Great sounds ... and built like a tank.
Trying to stay away from Ebay. I had two bad experiences with sellers. I am looking for just a 5.1 receiver for the basement. Next year i am going to finish it off and also get a projector. If I can find a cheap receiver to run 5.1 for a year or two then i will sink the money into an upgrade.