I use a stone made by American Metalcraft and like it very much. They're around 50.00 but they're thicker than the ones you get at Target or Wal-mart, which IMO makes a better pizza because of better heat retention, especially if you're baking several pies. They're made of Cordierite instead of clay. Much less prone to cracking. I leave mine on the bottom rack pretty much all the time.
As for stone, I've heard soapstone is great for this purpose but I don't know where to get it. It's probably very expensive too.
Some folks I know have used unglazed quarry tiles and fire-brick splits with good results.