[Troo]

As you don't speak Japanese your best bet would be to speak to the agencies you've already dealt with when booking gigs in Australia and Singapore and enquire as to whether they can represent you in Japan, or have contacts who can reccommend an agency which can do so.

Assuming you have maintained good relationships with your agency / agencies, and they're good, they should be able to assist.

Note that I'm speaking from the perspective of a writer who would approach her agent if she had this requirement. I am making an assumption that the music industry is just as insular and reliant on networking as the writing business.