If you wish to purchase Japanese music, which I would recommend as if you create the demand they will supply, there are some other choices.
First and foremost, JPU Record’s awesome catalogue is available here (as mentioned in the how to access page). Functioning as a distribution label, their CD’s are widely available on Amazon and HMV, but they also have their own online store.
There is quite a few Japanese record labels who distribute on the English iTunes. I won’t deny it can be hit and miss, but the range is definitely growing. Generally bigger artists in Japan are available over here, but there are some exceptions. I would recommend to keep checking if an artist you want appears, as I have seen albums go up in stages on there.
If you want physical copies, other than JPU, Amazon has some import CD’s but I will warn you they can be very expensive due to the import fees. Sometimes you can find cheap copies so is worth a look. You can also import from Amazon Japan.
If you are going to import there are a few sites but the one I would recommend is CDJapan (http://www.cdjapan.co.jp/). The cd’s are sold at the base price they are in Japan, with a decent range of delivery options. Of course it is still more expensive than buying CD’s here, but CD’s are more expensive in Japan than they are in the UK in general.