If you’re ever in Tokyo, go and check out Yellow Submarine in Shinjuku. It sells loads of role-playing paraphernalia in both English and Japanese, including some things you might not find so easily elsewhere.
I was there last weekend and bought some D&D miniatures to round out my collection, blank dice for some solo role-play and Gloom, a neat little card game by Atlas Games. While prices were generally high, it’s probably the best place to get games in Japan besides amazon.co.jp. The staff speak some English, but knowing a little Japanese is always helpful.
Besides role-playing games, you can also get boardgames and card games in Japanese, English and German.
You can find a map here, with the large red rectangle being Shinjuku station. Take the West exit. The smaller red block on the map is Yellow Submarine.
Let me know if you pay Shinjuku’s Yellow Submarine a visit and if you found anything interesting. I’d love to hear about your adventures.