I found a decent sized one at Bed Bath and Beyond. It was fairly cheap too. I think it's about a 16 x 14 square, perfect for my oven.
I bought the same one. It works great. I almost bought the All-Clad one, for $105, thinking it has to be better, just being an All-Clad, but there is just no way I needed the serving tray that it came with. Who serves their pizza on a 500 degree stone. And could you imagine cutting on one, by the time the first slice got off of it, it would have to be burned. On the same token, the $15 ones seemed to be a lil too cheap in my book. They were too thin, and I figured heat retention would be less. Very pleased so far though. Stone in nicely seasoned, and makes great pies. And the square size does give some extra play when loading a pie into the oven and allows an oversized 12" some room. When it finally goes, I do plan to line my bottom oven area with thick tiles, I would love to crank out a 16" or if there is a heaven, an 18". Now that is sexy in my book!