It's not something I have really looked for in the UK as I was lucky enough to spend quite some time working in Japan in the past. I've not punted there as the civvie opportunities were enough to satisfy my interest & needs.
Personally I thought they were on the whole quite disappointing, constant little moans & pretty unadventurous in bed but you would expect that to be different with a pro.
One thing that struck me about Japanese women is that there is two distinctive types, cute as fuck & pig ugly, fuck knows what they do with the girl next door types?
I'm no expert but I would think that a decent Japanese prossie would have a better earning potential staying at home so I would guess that most that advertise as such probably are some other nationality trying to cash in?
However I'm sure there is some exceptions to my thoughts its just a matter of finding them?