A good stone is never a bad idea. Besides fibrament, corderite is also a popular material and is pretty easy to find in the Old Stone brand at stores and the internet (amazon among others).
Regarding the tiles, you will prob not find them at Home Depot or Lowes unless you get lucky. I bet the sales folks there won't even know what you are asking for.
But if you locate an actual local tile store they will immediately know exactly what "unglazed quarry tile" is, and be your best shot at finding some. If you find any, you will prob just have to accept whatever size they offer which is no big deal. Just fit them together as needed.
You will really know you have found the right stuff when you verify their way cheap price. Maybe that is one reason they are hard to find = too inexpensive. Plus, their most common use is commercial kitchen floors and are seldom used in homes today I was told.
I found 6x6" tiles this way and bought 20 for $9.72 inc tax.
Here is an example of what you are looking for
http://www.emser.com/pdf/quarry_brochure.pdf, and you want the non aggregate or smooth type.