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I have heard Home Depot but I haven't seen them there. I went to Bed Bath and Beyond for mine. I have got a couple off E-bay if you have alittle time on your hands.
I have a pizza stone in the bottom of my oven and it works great. I don't know how it would work in a smoker though. I went to a store that sold and installed granite counter tops and asked for a remnant. He had a pile of stones left over from sink cutouts that he let me pick through. I took three of them, and gave two away as gifts. The cool thing is he didn't charge me anything. They were a little odd shape, but I cut them down with a cheap Harbor Freight tile saw. Just make sure that it is polished not glazed or coated.
I use a little cornmeal so that the dough doesn't stick. Works like a charm. I noticed that my oven doesn't cycle as much anymore either. I guess it holds a lot of heat being over and inch thick.
Some of the best pizza stones are made from Cordierite. Some can be expensive.
However you can pick up the exact same stones at a pottery store that has supplies for Kilns. I purchased a 15x16" 5/8" thick for $17+ tax. This stone is good up to 2100 deg F. The difference the kiln stone and pizza stone version, the kiln doesn't have a FDA safety sticker.
I use it in the weber kettle or my elect. oven. In the weber kettle I can get temps up to 700-800 deg. for short periods, and cook a pizza in 2 to 3 minutes.
When I purchase the Cordierite stone, the guy helping me, told me he sells them to various pizza and other restaurants as replacement stones in their ovens.
Search my name and you will find several posts on making pizza in a weber kettle.