Anyone can access the Japanese iTunes store from within iTunes on their PC or Mac.
Click on the “iTunes Store” button on the left side menu in iTunes. Scroll down to the bottom of the store and you will see “My Store”, with the name and flag of the country’s store which you are in at the moment. Click on the arrows to the right side of the country name and a dropdown menu of all the international iTunes stores will appear. The Japanese store is at the very bottom of the list, written in Japanese. Clicking on this will take you to the Japanese store.
You do not need to buy an iTunes card before you browse the Japanese iTunes store. Anybody can access it, however if you wish to download from the store, you will need an iTunes card, or a Japanese credit card.

Once you have found something you like, you can get your Japanese iTunes cards from JPopMp3s.com