Ebay have a fair few most of the time. I am in no way a Mac aficionado, but picked up my MBP and Mini off there for a bargain. Both were upgraded and came with loads of other kit.
My Mini is an 09 that I picked up last year for around £300 (inc magic mouse, another kb, extra 2Gb and some other bits). Mint condition and does everything I need.
And the MBP was £2k of stuff that I got for £650. 18 months old too. Bargain.
Depends what you need it for though …