ebookjapan is an online bookstore in Japan that I have used for many years to buy and read Japanese books and manga. It used to be the one site that allowed foreign credit cards. In the past few years, eBookJapan has merged with Yahoo! Japan to make use of PayPay and Yahoo! Wallet or Yahoo! Card and other payment processes. I thought for a while that I wouldn't be able to use it anymore, but even after the merger I was able to continue to pay for purchases with a Visa card. Just remember that there will be a small foreign exchange service fee added to your credit card bill.
It tries to ask for a Yahoo! Japan ID (which I had gotten a couple of decades ago) as part of setting up an account or a cell phone number, but if you read deep into the help screens, there is an option for people who have neither.
That will take you to the form which will let you enter your regular email address to register. Once you are logged into Yahoo! Japan, you can click on the link to go to ebookjapan or you could go directly to ebookjapan and login there.
By clicking on your name on the right hand side, you can register the information needed for your backup email, credit card info, and whatever else you choose to give them.