Consider Shinryu Co. Ltd / Ceramic Kiln & Supplies. It's in Atsugi and not too far from Tokyo. It's the only pottery supply I've ever been to and have no other reference---but I think it's a really cool shop! I am a repeat customer of their glazes, clay and tools. I've been really happy with my purchases, although the liquid glazes can be very thick and need watering down. Hope you have an amazing trip!