Kawamasu Camera, Asakusa.
This has to be one of the hardest-to-find stores in the world. It's located in the grid of narrow buildings in Asakusa, one of several camera stores to grace the east side.
You really can get turned around looking for this place, I've gotten lost almost every time I've tried to find it. The key, I believe, is to start with Kuramae station on the Toei Ooedo line (exit A5) and head north to the second semi-major street. Then cautiously head west. It's located in Kotobuki 3-12, you can find it on my map of Tokyo camera stores.
I recommend digging the place up. They've got tonnes of stuff of every description.