How can I download an Itunes song only available in japan

You will need to be in Japan with a Japanese billing address and iTunes account to be able to buy from the Japan store. You can try requesting that the song be added to your country's store, but ultimately it's the record company that will have to grant Apple a license to sell it there : http://www.apple.com/feedback/itunes.html

Is it available on other music purchase download sites that you have access to >