Got mine on Ebay, and you usually have to be careful like with any purchase of a used item: ask for detailed photos if you can't judge the state of the item from the photos that are there, ask questions about things that were left out in the description, look for sellers with very good ratings and look at their reviews, see if you can return etc...

Amazon also sells refurbished ones, but so far I've seen the best deals on Ebay.

EDIT: important, make sure the BIOS is unlocked.