In-app purchase from Japanese app So I tried to buy some in-app offers from a app that I downloaded in a different app store region, after putting in my billing information it won't go through. I'm using a US debit/credit card. 

Is there anyway to go around the problem like buying a iTunes card then use it for my iTunes account to buy in the in-app offer?
Yes that is possible. You can buy gift cards from other country iTunes Stores. 

One quick search and I found https://japancodesupply.com/, where you can buy Japanese iTunes Gift cards. When you order it takes approximately 1 minute to send you an email with your code.

There are many other websites that offer iTunes Gift cards from other countries. So do a quick search, check the credibility of the website and buy a giftcard.

I'm not affiliated with japancodesupply.com