There is a directory assistance number in Japan.
You can try asking a friend in Japan to dial for you (i don't think you can dial from overseas) - i believe the number is 104 when dialing from a cell phone in Japan.
it works, but you need to know the city where the person is living, and many people are unlisted.
BTW, u can also try this private investigator company in tokyo (probably really expensive)
(from US dial +81-3-3403-5151)