Definitely Roppongi for 'gaijin' ladies (North American, Russian, British, Aussie etc) - though most are 'hostesses' and not SP's.
Also, I was back in Tokyo this past November and found that Roppongi was brimming with Nigerian pimps and promoters - at least a dozen outside on every block aggressively pushing their 'product' on passers-by.
AVOID. So many scams - some of which are really sketchy :/
Shibuya is a vibrant, 'youthful' area to go dancing/shopping/eating/clubbing (and is full of 'love hotels') but IMO, is more of a 'hang with your buds' part of town than a 'get your poon on' kinda place.
I would recommend Shinjuku (Kabukicho, in particular) - which is essentially the 'red light' district of Tokyo.
If you wander the streets, you are bound to find plenty of parlours there
Ginza is also a consideration, if you are interested in more high-end Japanese ladies.
Good luck!