If you are working in a urban area, so your subject is surrounded by rental units throughout the neighborhood, your answers and all your rental data are sitting there for the taking. It just needs a person willing to work to get it. I made up a tenant interview form and I go door knocking. I drive the area just to spot similar rentals to my subject, I get out and interview tenants. You do not need the same number of units per building, just similar units in any building. Even one tenant only in a fourplex giving a successful interview that can be used for rental data for a subject duplex is gold! Yes, it can take a couple of hours. But I have also been done in half an hour. I have found it's rather fun myself and I am generally very well received by tenants. Once I have even four successful interviews from comparable unit properties (I often try to get 8 or so) I can now use this with, and compare against, MLS reported data.