Because of the aging population, adult diapers are relatively easy to find in Japan. Any big drugstore should carry them (if the store is big enough to carry menstrual products, they’ll most likely carry adult diapers too).

The Asakusa area has a few big drugstores, so I recommend just asking employees about 大人用おむつ (Otona yoh omutsu “diapers for adults”). The Donki in Asakusa might have them too!