If you want to get access to the JP kindle store, make a new account on amazon.com, and an account on amazon.co.jp if you don't already have one (use a different email address than your normal US account). Link your new .com account to your .co.jp account. Then you can set your location to Japan (you'll need a Japanese address) and you should be able to buy books even using a US credit card.
I live in Japan so I have an actual address that I can use, but I can't imagine just using a random address won't work (I make no guarantees though). I can confirm that I can use my US credit card just fine on the site to buy Japanese kindle books.