I have picked up mobile hotspots here in the US for my travel to Asia. I have used Xcomglobal . They deliver the mobile hotspot by Fed ex and give you a envelope to mail out on your return.
I think it was $12.95 a day for unlimited DATA. On my trip to Korea and Japan this was incredibly useful. As soon as well landed. I was able to turn on my mobile wifi and start texting . I have also used this company in Europe as well.

Not sure the rates in Japan, but check out Xcomglobal website.