Gamertell has posted an article that explains how to purchase Japanese game music via iTunes.
From the article:
"Unfortunately, direct downloads of video game soundtracks and Japanese rock or pop music aren't that prevalent yet. For many people, the best way to go is straight to the source - iTunes Japan.
The Japanese iTunes store has tons of music waiting to be downloaded. Tracks from your favorite video games or obscure Japanese artists. There's just one catch, you have to have a Japanese iTunes account to access all of it. And, to have a Japanese iTunes account, you must have a Japanese mailing address and a Japanese credit card or iTunes music card. Fortunately, creating an account isn't a terribly difficult task."