I don't know about Canton, but the best way, by far, in Beijing, is to go to one of several bars where working girls hang out. You can check them out, chat with them and negotiate with them before deciding to take them back to your hotel room. Some hotels are reputed to be sticky about bringing ladies back at late hours, But I never had any trouble.
You should join internationalsexguide.info and read the reports there. That's how I knew where to go in Beijing. Remember, Canton is listed under its real name, Guangzhou.