@yawnz
you need a JP Amazon account, so take a burner email (or a real one if you please), put in any valid address in Japan as your SHIPPING ADDRESS (or your own, but I personally haven't tried that, I've been told it works but can't vouch), and set all that up
if you have an international credit card, add that (with your proper BILLING ADDRESS), then on the .jp front page, hover over 'Your Account' and select 'Manage your content and devices' -> go to 'Settings' -> In the Country Settings, change your location to Japan
note that if you have a US amazon account and want to use the same email, you need a different password, otherwise you'll just get logged into your US amazon account instead of your JP one
and if Amazon asks to link to your US account, don't do that
(sidenote for any looking in, if the above + $7 is a low enough bar of effort to go through, give it a shot and burchase to support the author)