Active street prostitution areas in Copenhagen use to be Skelbaeksgade and Halmtorvet. I am not sure if you can still find any hookers from those areas.
If you like black prostitutes, then go to Istedgade in the part towards the Main Train Station. Some of the side streets along Istedgade have Asian massage parlors and Danish erotic massage parlors. Overall Copenhagen has never been best place to find streetwalkers. Most street prostitutes in Copenhagen I seen have been junkies though.