I know that it's possible to access the Japanese iTunes store from iTunes on a Mac or PC (I haven't tried to access it on an iOS device). First you'll have to create a Japanese iTunes account; there are instructions on how to do this in the idolmaster mobile i thread. It's been a while since I did that, but I think it involves switching to the Japanese store, clicking to download a free app in the store, then fudging a street address in Japan. Then you'll need a Japanese iTunes gift card. Once you've purchased and downloaded the songs, I guess that they'll play on an iPad, iPod or iPhone without jailbreaking. Wikipedia says that the Japanese iTunes store stopped using DRM for music earlier this year. Even if the songs still had DRM, I think it's not difficult to authorize your iOS devices to play them. Note that I haven't actually done all this yet; I still haven't bought a Japanese iTunes gift card.