Erotic massage studios aren’t the most popular way to spend leisure time in Istanbul. The city was filled to the brim with salons offering erotic massage (erotik masaj) about 10 years ago, but then they slowly started to disappear. There just might not have been a market for that. You can still find the occasional happy ending here and there. But they are few and moving from one place to another quite often.
In all honesty: you're better off looking up an escort with massage skills on one of the local or national escort directories and classifieds sites.