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.

ProviderServiceOnlineStore
mineo Prepaid SIM for Japan Travel Store List
IIJ Japan Travel SIM powered by IIJmio Store List
eConnect Japan Japan Prepaid SIM WEB
b-mobile VISITOR SIM WEB Store List
Sakura Mobile Japan Prepaid Data Sim Card WEB
So-net Prepaid LTE SIM by So-net Store List
U-mobile U-mobile Prepaid Store List
WirelessGate Wirelessgate Prepaid SIM Store List
Yokoso Japan Yokoso Japan SIM WEB Store List
FREETEL Prepaid Data SIM for Japan FREETEL SIM Store List
NTT commnucation Prepaid SIM for Japan Store List
BIGLOBE BIGLOBE NINJA SIM WEB Store List
Telecom Square Wi-Ho! Prepaid SIM WEB Store List
TRAVELTECH SIM TRAVELTECH SIM WEB
JPSIM JPSIM WEB Store List