It seems that several years ago everyone was buying unglazed quarry tiles at Home Depot & Lowe's. Word on the street, though, is that these companies ceased these sales in order to force customers to more expensive tiles.
But, now, I feel like I'm searching for Nessie? A local tile place is helping me look...they'll get back to me soon. But, man, these things are really elusive.
(I've been making pizzas for a matter of weeks - unfortunately, even a stone is out of my budget right now.
Pic related: my current laughable "stone." It can fit like...an entire 7 inch pizza. I use the goggles in case it explodes in my face.)