I found this book today and it is so interesting! You get to know the story and inspiration behind famous paintings.
There are a lot of prostition hidden in these motives. For example, did you know that ballerinas didn't have much status when Degas lived? It was a very common entry way to become a prostitue!
Also in this picture by Seurat you can see signs of prostitution. One prostitute is fishing on the left side - a way to camouflage what she was really up to. It was a common thing along the Seine at that time, and they were aiming for the soldiers (you can see a couple of soldiers in the background).
To make a painting with prostitutes in Madrid today, I guess it would be motives of articles in the newspaper (a lot of them in El Pais, one of the most popular Spanish news papers), in the park Casa de Campo, near the metro station Sol where they are waiting in their mini-skirts and high plastic heels, on the Internet, bordells of course, or at the high-end discos and private parties where you find the luxury prostitutes. It is still such an industry.