I'm looking to do low relief backgrounds myself. I think at this point other than scratchbuilding, it's going to be store-bought whole structures that I cut the fronts and backs off of.
For photo-based ideas, I experimented with a Google street view that showed the front and side of a building and a little bit of the back, with enough there to Photoshop the rest of it. It might be OK for deep background, but the lack of relief on the façade was fairly unsatisfying.