i noticed a forum post on baking stones that has been closed to replies, and i've found a decent solution that doesn't require a $60+ purchase.

alton brown suggests purchasing an "unglazed quarry stone" which, while being rather vague, leads you on the right path. he is talking about what are called saltillo tiles, which are approximately 20"x20"x3/4" and red adobe in color. they sell them at the home depot in my area (southern california) for $1.50 each but i've heard they're much cheaper elsewhere. if you get lost, just ask for the saltillo tiles, they'll know what you mean.

two of them cover the bottom of my oven perfectly and make delicious pizzas and crusty boules. i find they make an excellent substitute for a baking stone, especially if you can't afford the fancy williams-sonoma one.

on another note, make sure that you season these tiles before using them. nancy silverton suggests seasoning your tiles at a low temperature for several hours or overnight; otherwise, they will crack when you turn the heat up to the necessary 450/500 degrees.

hope this helps, happy baking!