https://www.herrickstamp.com/new_issues.phphttp://newstampissues.comSilhat, I'm not quite sure what stamps you are looking for from Japan, but if you want general new issues sent to you every few months, the above two new issue dealers can help you.
I've used Herrick for many years. They're very good, though I do not currently buy from them because I'm desperately trying to cut back on buying new issues so I can buy older stamps. The other seller, Davidson, I have never used, but I've heard good things about him. I'm fairly sure both of them can supply you with new Japanese stamps on a regular basis. Herrick marks up the price of stamps by at least 50%. I'm not sure about the other seller, but I think his prices for new stamps may be slighter lower.
The Japanese postal service seems quite strange about not selling stamps to collectors outside Japan, but they seem to have always done that, in my experience anyway. There may be language issues or perhaps they simply decided they didn't make enough money selling to "gaijin" (foreigners). The U.S. also does not make it easy to buy new issues. You have to search the USPS website or the USPS catalogue, but they don't provide annual subscriptions to stamps like in many other countries -- or at least I don't think they do. European countries and some others have a long history of selling stamps to collectors which is why it's frustrating to see some not make much effort to do so. And again, I have no idea why they don't.
Maybe one of the above dealers can help you. Check their websites.
It's also possible to trade stamps with a Japanese trading partner. You send them what they want from your country and they send back new issues. But I have no idea how you arrange such a thing.
Finally, last suggestion, you might consider joining the
International Society of Japanese Philately which I've been a member of for many years. It publishes an excellent journal. In the journal (published in English from the U.S.), there may be listings of collectors or dealers who sell modern Japanese stamps.
http://www.isjp.orgLet's hope you can find a good source for new Japanese stamps. Bon chance!