I bet you can buy it in Japan, along with several other maps and atlases. Go to the Kinokuniya store near Shinjuku Station. Exit via the New South Exit and follow the instructions for getting to Takashimaya Times Square and Tokyu Hands. Kinokuniya is the last store in the row, which overlooks several parallel railroad tracks.

This link contains a map. Kinokuniya has a whole floor of books in languages other than Japanese, mostly English, and an extensive selection of travel guidebooks.

http://www.kinokuniya.co.jp/c/store/.../shopinfo.html