if you have more than one pax, and need to use alot of data.
a pocket wifi is more than useful.

can try klook and kkday. can apply some promo code and get 10% off.

If it is u alone, maybe u can try docomo travel sim. but those have limited data.

errr... buy what?
try to help you by answering your question.

but u never even bother to reply which option.

try klook.com or kkday.com to look for sim card to be delivered to u in Malaysia.(unsure of their selection)

But if you are in a rush to get it, best for you to get the the airport in japan to buy your sim card.
there are vending machines around there and also there are also kiosks there that rent wireless wifi and also travel sims.

also alternatively you can book online
https://www.globaladvancedcomm.com/

collect it at airport.

have a good trip