I like Microcenter for buying "older" Macs. They tend to carry models older and less expensive than those offered in Apple's refurb store. You have to be careful, though. Last time I checked, Microcenter was still selling plastic Macbooks (which can't even run Mountain Lion). If you're in the DC area, there are 2 MC locations, one in Fairfax, VA and one in MD called "Beltway/Rockville".
Of course you could buy from an individual on CL or ebay but I don't recommend that because if they get wind you are only in the country for a short time, you risk getting cheated.