FYI, I need 3-4 stones and planned on buying the Cordierite Shelf - 18"x18"x3/4" from http://www.axner.com
. With shipping and packaging cost, 3 stones was about $160.
From an local restaurant supply house, I found 16.5" (5/8" thick) round stones made by Outset for $33/piece. They gave me a 10% discount for buying three, total cost was $97.
For cooks/baker out there, I strongly recommend a visit to one of these supply house if you live in a large city. Some of their stuff is cheaper than WalMart yet much higher quality.