Hmm, you should probably go to Docomo or SoftBank. Docomo promises that you can get a signal almost anywhere. However if you're only going there for a bit, you should get the cheapest phone you can get.
I only know about SoftBank, the cheapest that you can get is Samsung 821sc, or the SHARP 831sh. But if you can afford a really expensive one, you should get the Aquos phone, or Aquos Shot.
You can also get yourself a pre paid phone, however this is a bit of a hassle, because of the exchange rates.