Your best bet will be to change it at the airport in Japan after arriving. The exchange rate is much better than any bank or airport in the US. That's about the best you can expect.
When I went to Japan last May, the Dallas airport exchange rate was a full 4 yen higher than I was charged in Japan. If the spot rate is say 110yen/US$, Dallas would sell you yen at 104 yen/US$ while Narita airport would sell you yen at 108/US$