I used this for 7 days, but you will need the 15 days. You get 220MB at 4G speed renewed each day. It drops to a lower speed if you used pass 220mb
7 days is ¥2500 and 15 days is ¥4000
You can get it only from the vending machine at KIX - just go all the way left when you get out of Customs. I think there is a Starbucks or Coffeebean just opposite the machines. None of the shops sells it.

https://umobile.jp/lp/prepaid_eng/

you need to choose between nano and micro sim. dunno why japan so low tech, why they dont have one card for all type like Thailand. also choose the number of days. There is a money changing machine next to it but if you got a bigger bills, then this machine cant change for you. instead look behind you and you will find another machine that can change 5K, 10K yen

There are other vending machines selling other brands but for me this is most suitable and cheapest