I keep a (Vodaphone) prepaid phone (along with a prepaid phone card) that I picked up at a Costco here in the glove compartment of my car for emergencies. At the time, it cost a little less than 5000 JPY for both the phone and included a 3000 JPY prepaid phone card. But as I recall, Costco also needed to verify Japanese ID before they would activate the phone.