I've bought off Craigslist with good results. A lot of the bikes I see there are from people who bought a nice bike and then quit riding because the trails were too far away, their friends got out of it, etc.
Talk to John Moore ("Big John") at a group ride. He might have a bike for sale that will fit you.
Also watch E-Bay, I did well there too.
In my opinion, keep an eye out for a Cannondale Super-V (NOT the design with the carbon swingarm). They're good bikes and can be found at very reasonable prices. They can be built with anywhere from 3" to 6" of travel front and rear.
Whichever bike you get, you might have to start with a bare frame and build up from there. You can get great deals on lots of perfectly good parts because someone upgraded to fancy expensive stuff and is trying to recoup some of their costs.