Though Pakistan legally does not allow prostitution, it goes on in private houses. You don't see any advertisements outside. My favorite places in Karachi are one called Salma's and Farida's. Most of them speak english and urdu. They don't accept credit cards.
Cash is king.
You can't find them in newspapers. In Karachi, if you ask rikshaw driver they will take you there. I have couple of friends working in Police Dept. They directed me to houses.