If you're on a tight budget, buy unglazed stone tiles from a tile store.
If you're in the market for a regular baking stone I recommend the Villa Ware stone. It's 14"x1"x1/2". This is a SERIOUS piece of stone. Price: around $40. King Arthur's Baker's Catalogue online sells them for $35 plus shipping. I've bought stuff from them for years. Top notch.
DO NOT waste your money on one of those skinny little round or rectangular ones. They're typically around 1/4" thick and they'll break soon as look at it. That, and they're too small.
I've had my Villa Ware stone for going on 8 years. It stays in the over all the time on the bottom rack on the bottom of the oven. when not baking breads/pizza it helps keep the oven at a constant temperature.
Every once in a while I scrub it off with stiff brush and plain water...never use soap...and slowly dry it in the oven.
You get what you pay for. Buy this one. You're worth it.