When I was doing some research for my 5 and 10 cent store, I ran across some interesting sites for your downtown buildings.
Of course the one that started it was the “Build Your Own Mainstreet” - http://www.illinoishistory.gov/ps/construct_mainstreet.htm They have cutout models for HO, but you can easily upscale them for large scale.
The Mesker Brothers provided prefabricated architectural elements which ranged from individual components to entire building facades. You can download the Mesker Brothers Iron Works catalog from 1906 here: http://www.illinoishistory.gov/gotmesker/Mesker_Catalog.pdf
There are 16 examples of Mesker Buildings on this page: http://www.illinoishistory.gov/ps/gotmesker2.htm