I'm not sure about down there, but places like Juntendo, Nafco, Comeri, Conan--like a hardware/lumber store. They also have furniture...
I ordered my Naniwa stones from
here, they have free shipping over 5,000 yen and they have the full range of Naniwas for a LOT less than you can get them in the states (the 12000, unfortunately, is not sold in Japan). You can get the 10mm thick 8K and 3K (or 5K) together for less than $60. It's all in Japanese, and they don't take credit cards, but there are ways around it.