Your best bet is Apple Refurbished Store at Apple.com. Apple also has an eBay store where they sell refurbished products even cheaper than their refurbished store. iPad 3 is your current best deal. I believe they just started selling Refurbished iPad 4's at Apple.com.

Difference between iPad 3 and 4 - the 4 has a better camera, better processor and the Lightening dock connector.