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

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
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.

• Prepaid SIM card vending machines at Narita Airport
• Prepaid SIM card vending machines at Kansai Airport
• Prepaid SIM card vending machines at Haneda Airport

Provider	Official Site
CDJapan Rental	https://rental.cdjapan.co.jp/
IIJ	https://tr.iijmio.jp/
WirelessGate	http://www.wirelessgate.co.jp/presim/en/index.html
JP Mobile	https://www.jpmob.jp/prepaid-sim/product.html
BIGLOBE	https://biz.biglobe.ne.jp/ninjasim/en/index.html
Telecom Square	https://sim.telecomsquare.co.jp/
JPSIM	https://www.jpsimshop.com/
eConnect Japan	https://www.econnectjapan.com/products/sim/