Prostitution in Turkey is legal and organized by the government.
Prostitution in Istanbul Escorts Hostess Bars Brothels That said, state-run brothels (genelev in Turkish) are normally reserved for locals. Foreigners who visited them mention paying an extra fee, having to bring their passports and going through security screenings. The main red light area is in Karaköy district, with 20-30 girls, reportedly quite old. There are other locations in Tarlabaşı and Fatih.
Outside of these, there is some freelance prostitution on some streets and some upscale discos like Reina or Sortie. You also have hostess bars, most of which are tourist traps. You should be careful to avoid them, especially if you are not familiar with the country. These places are run by mafias who specialize in extorting money from confused tourists. They advertise cheap drinks from outside, then present you an outrageous bill that you cannot avoid to pay. Be smart, don’t follow a stranger met on the street to a bar with big neon signs.
If you are really in a desperate horny mode, you have two and a half options:
The first one is to visit some of the bars in Aksaray with Russian/Ukrainian/Romanian hostesses. You have to pay about 60TL entrance inclusive of one free drink. Inside, there is a small dancefloor where you can chat with girls and negotiate a price. From what I could gather on internet, the price is around 100 euros for a short time, plus 50 euros for a short time hotel nearby (the girls usually refuse to accompany you to your place). Recommended venues include Balance Disco, Serenda Hotel, Bacardi, Gold Can and Big Pub.
You must be careful as this unofficial form of prostitution is illegal in Turkey. Police may raid dubious hotels from time to time.
The second option is to take your chance with online istanbul escort service. Many people looking for paid sex in Istanbul will use services like Ela Şimşek (reachable via facebook) or escortnews.eu. The price is usually between 100 euros and 300 euros. Beware of fake profiles.
The third half option is to try a massage parlour with plus plus services. From the information I got, Relaxium Etiler is one of the very few spas offering special treatments for men in Istanbul. Istanbul Escorts There was another one called Princes Spa in Osmanbey but it has closed down a couple years ago.
If you need a girl-friendly hotel, please check my review: Best Girl-Friendly Hotels Near the Nightlife in Istanbul.