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.
I have lately been using a brand from japan called liv dry, the diapers themselves are not that absorbent but they sell pads that go with the diaper so you can make it as light or heavy as you need. Personally i use a large size diaper and either a super or hyper absorbency insert pad, with a hyper pad the diaper will not leak unless it is absolutely full. As far as where to get them in japan i dont know but here in the u.s they are sold by a place called tye medical.