I’m a big Garry’s Mod fan and a fan of the mappers on Facepunch. I thought I’d post this site I’ve known about for a while:
This is the Library of Congress American Memory collection. Part of the collection is the Historic American Building and Landscape Survey’s which records historic structures in all 50 states as well as territories.
The site lists buildings by state or subject. 90% of the entries have pictures and quite a few have the plans/drawings of the structure. The appear to be adding to it quite often so some stuff hasn’t been digitized.
I felt that a lot of good ideas for maps could be gained from this site.
I hope this helps