I'm 24/7, and went to japan for a week, about three years ago. They DO NOT have evenn half-decent diapers in any of the common pharmacies. You have to scour around for a medical or compounding pharmacy- which are often hidden away, and far and few in between. And even then, the diapers they carry are mediocre.
Like in the US, you are much better off ordering diapers online and having them shipped to you (hotels may charge to hold them for you). Alternatively, you are allowed one free checked medical bag on most all flights. Just pack all the diapers you will need, and bring them with you.