Okinawa is good for their massage parlors.
Some Buy-Me-Drink-Me bars have a brothel in the back.
The mainland is a little harder to find service.

Japan is VERY safe to travel alone so don't worry about dark alleys. Seriously! I've seen little girl walking around after dark.

Ask a taxi driver for a relaxation for men spa and you'll probably get lucky.

Massage parlors will run you about $80-$150.
FS goes for about $200.

also ask for places like Whisper alley or Soapy towns.

Asian women will treat you a lot better than more western women will, so have fun.