If you scratch build, check for building material in the model railroad section of your hobby shop. 1:48 scale is O gauge scale. The model RR section has a couple of different forms of sheets of building materials, for things like brick and concrete blocks. A simple box of any material, sheathed in those sheet overlays, and window frames (also available in many shops) will go far in helping the build.
If local hobby shop doesn’t have enough of that stuff, check out Walthers online (and maybe send for their catalog).