There are in fact a few schools that specialize in praying mantis around Tokyo (as well as an old guy or two who just practice on their own in a park somewhere) - unfortunately, off the top of my head, I can think of one that is on the OTHER side of Tokyo, so it would be a 2-hour train ride for you (one way). Still, you might want to check it out - the teacher might also be able to point you towards something closer, but I don't know (note that I've no idea what these people are like).

They teach taiji, liuhe, and taijimeihua praying mantis (it does seem to be Japanese-language only).
http://ichikawa.laotanglang.jp

(I will try to find other mantis schools for you as well, stay tuned.)