I picked up a big, thick rectangular stone from one of those factory outlet places, cost me about $20. Still going strong almost a year later, even when putting frozen pizzas right on a hot stone.
Just remember to leave some space around the edges so air can circulate from the bottom heating element and the rest of the oven. I hear that you shouldn't put high-fat stuff like cookies right on the stone either, the fat will be absorbed by the stone and turn rancid. I leave mine in 24/7, often putting pans right on it. It keeps everything at a more constant temp and helps cook the bottom of stuff so the top won't burn while the rest of the food is still cooking. You will need a pizza peel (that big paddle) for pizza though, as the stone is very heavy and hot, making it not very removable. Those unglazed tiles are usually too thin and wimpy for serious baking. The stone needs to be pre-heated for 30-45 minutes so it can soak up a lot of heat first. It has the added benefit of drawing moisture from dough, making a crispier outer crust (so if you want a chewy crust, don't set it right on the stone). I consider a stone to be a vital part of a stove, makes baking a lot easier and more predictable.