In Japan, there are lots of shops that sell prepaid SIM cards.
Among them, we picked up ones offering foreign language support.

YODOBASHI CAMERA
Yodobashi Camera
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

BIC CAMERA
Bic Camera
Their flagship store in Ikebukuro is the biggest.
Meanwhile, Yurakucho store located in Ginza is very convenience and close to the Palace area.
Ikebukuro Flagship Store Yurakucho Store


There are some shops and vending machines of prepaid SIM cards at airports in Narita, Haneda, Kansai, Chitose, and Naha.


ProviderOfficial Site
mineohttps://travel.mineo.jp/en/
IIJhttps://t.iijmio.jp/en/
b-mobilehttp://www.bmobile.ne.jp/payg_sim/travelers_to_japan.html
So-nethttps://www.so-net.ne.jp/prepaid/en/index.html
U-mobilehttps://umobile.jp/lp/prepaid_eng/
WirelessGate http://www.wirelessgate.co.jp/presim/en/index.html
Yokoso Japan http://www.yokososim.com/
NTT commnucationhttp://www.ntt.com/personal/services/mobile/one/visitor/en.html
BIGLOBEhttp://ninjasim.jp/en/index.html
Telecom Squarehttps://sim.telecomsquare.co.jp/
JPSIMhttps://jpwifi.co.jp/
eConnect Japanhttps://www.econnectjapan.com/products/sim/
MAYA SYSTEMhttps://mayass.com/prepaid/