Admittedly a while ago, but when I was there I had no problem finding action in Shibuya. One at a massage venue they were happy for me, a gaijin, to enter, and another one of the street hustlers offered herself instead of one of the girls, she took me to a little set up she had, like a studio flat. I didn't spend much time in Shinjuku that trip but had done previously with some success, but remember to ALWAYS be super careful around Kabukicho. I avoid punting there because it's just not worth the risk.
When I was there in the late 90s there was an area in Yokohama called Koganecho. It was 150+ small brothels in a series of lanes and alleys with girls of many different ages, sizes, nationalities etc. The city cleaned that all out in time for Yokohamas 150th anniversary though. Isezakicho has some soaplands off the main drag, not sure how foreigner friendly they are. Remember, the yanks have an Air Force base west of Tokyo and a naval base in Yokosuka, and when their boys have base leave they can sometimes act like complete and utter fuckstains, allegedly, and making sure the locals know you're not one of them can help your cause.