I think that your best bet would be to enroll in a college or university. You can do an ISEP exchange through just about any school that you go to and they offer exchanges to Japan. The cool thing about an ISEP program is that they'd be able to work with you to transfer the credit back to your school (through your school's study away office or registration office). You might check out this site for information on the ISEP exchange to Japan:
ISEP - Member Sites