Sun, Catie, anyone else who is interested in Griswold or Wagner cast iron pieces, I recommend the Wagner and Griswold Society -
http://www.wag-society.org/
They are passionate about their stuff, and can steer you clear of fakes on Ebay. You can learn a lot from them, and if you would like to actually feel and touch before you buy, maybe there are members near you who are selling things. You learn a lot from the forums, but there are parts of the forums where you need to become a member of WAGS to view them.
Some of them get their cast iron in big lots, keep the rare ones and sell off the rest. You don't have to pay much for a piece you're going to cook with, it doesn't have to be very rare and in mint condition as far as I'm concerned. If it's been well used, it's been well loved. Just remember to check that it sits flat and there are no cracks. Plus I believe that a WAGS member wouldn't steer you wrong, their reputation depends on it!