The real estate agents put up signs that can be seen from the street in the windows of the apartments that are for rent/sale. I do not know how they handle the apartments that can not be seen from the street. The real estate agents advertise the available apartments that are for rent/sale with photos in the windows of their offices. The Clarin Newspaper lists apartments for rent/sale in the classifieds and they are organized according to Barrio.
Hope this helps
John