I just bought a 2008 2x2.4 (eight core) a few weeks ago from macofalltrades.com. They seemed to have much better prices than other avenues (PowerMax, LA Craigslist, eBay), and so far I'm pretty happy. Shipped nicely, though I did have to buy my own OS install disks as it didn't come with any disks (Leopard pre-installed). The included 90 day warranty was enough for me to make sure that there aren't any shady things going on with any of the components (1-year warranty available for purchase, too). For what it's worth, the case arrived in beautiful condition- hard to believe it was used and nearly 4 years old.
Anyway, it's an option. Don't see anything newer than 2008 on there right now, but it changes so it might be worth continuing to check.
Good luck.