In fact Japanese sex workers are hard to find anywhere in the world including Japan unless you are Japanese. Places that advertise Japanese girls typically actually employ Koreans, Chinese, or almost any east asian type other than Japanese.
In Japan, they will not even let foreigners in to what are called "Hostess Bars" ,a place where you pay to sit in a booth with cute Japanese girls and drink all night. Any such place where a foreigner is allowed has girls from the mainland of asia and sometimes eastern europe.
There are places in Japan that foreigners can go for sex, but it will not be with a Japanese girl.
I never bothered with Japan because I was always on a stop over on the way to Bangkok.
To my knowledge, most of the Asian girls in the RLD are from Thailand or the Philippines and maybe China.