In Japan, there are lots of shops that sell prepaid SIM cards.
Among them, we picked up ones offering foreign language support.
They have a wide variety of prepaid SIM cards, and for buying the best one for your purpose,
they have "SIM Counter," which gives you an advice on it.
One of their largest stores is located in front of Akihabara Station.
Yodobashi Camera Multimedia Akiba Store
There are some shops and vending machines of prepaid SIM cards at airports in Narita, Haneda, Kansai, Chitose, and Naha.