*Note - I am a Sprint employee*
Sprint will not work in Japan, nor will the average "phone off the shelf" for any of the carriers in the US. Japan uses technology that is totally different from everyone else. Please
read this, it is an article that discusses, in depth, what works, what doesn't, and how you can get a phone. It looks like your best bet is to rent a phone, however, it does cost an arm, and most of a leg.
Here is a website offering aircard rentals in Japan.
Your better bet may be hoping for wi-fi access.