Try www.globaladvancedcomm.com They rent a mifi that arrives at your first hotel in Japan, with a post paid return envelope you use to return the device from whichever city you leave Japan from. We carry the mifi on our day trips to provide wifi for our phones and also use back in the hotel for the MacBook or iPads. Depending on which device you rent (various speeds are available), you can connect up to 5 or 10 devices.

We've also tried telecomsquare.us, but they send the device to you at your home (in the US) and you return it once you get back. If you are going to any other country besides Japan on the same trip, you pay daily rate until you return home. Their devices are just as good however, and we encountered no problems with their service. They even give a discount if you are flying ANA.