Trusted computing vendor MacSales.com...
View deals + wish lists at B&Photo...
Buy anything at Amazon.com...
Buyer's Guide: Ordering a Refurbished Mac Pro
After ordering your Mac Pro, see step #2 of the Buyer’s Guide page for the parts to make it “scream”.
Ordering a refurbished Mac Pro PERMALINK
The trick here is some patience. Apple usually refreshes the refurbished offerings about once a week, so if you don’t see the model you want, check back each day. The models offered might or might not have exactly what you want (like Airport wireless), so if you have specific desires, you might have to buy new.
For more on buying refurbished, see Buying a Refurbished Mac.
When you go to the refurbished store, click the link at left to see relevant items eg Mac Pro:
As of February 201, the 2.66GHz quad-core 2009 Mac Pro or 2.8GHz quad-core 2010 Mac Pro are your best deal.