I know of 2 places that you can buy then.
First, the aviation museum shop on the 5th floor of Terminal 1 in NRT sells them.
Second, there is a very small pilot shop in tokyo that sells them and has a website.
–P•¶‘—Ño”Ŕ̔„ . They are about 5,700 Yen and the largest size is 27cm. I believe they are at the Shimbashi stop in Tokyo and their website says they have internet ordering available, but my Kanji isn't good enough to figure out what they are trying to say.