Here are a few links. Quality varies. Scale shown where known.
Greenfield old west style building links on this page
https://modellcom.blogspot.com/
From the Wayback Machine - an American Bank (low relief but easy to convert) - it looks like N scale.
https://web.archive.org/web/20150921113943/http://miniatures.about.com/library/printibles/bank2.pdf
A Barn with political slogans. The barn is HO scale 1:87.
http://www.ss42.com/pt/obama-barn/Obama-Barn.html
From the Library of Congress. A model of Plymouth Church, Brooklyn, N.Y. - scale unknown - images in different sizes and file formats
http://www.loc.gov/pictures/item/2006677655/
A church - no scale information
https://web.archive.org/web/20081007202544/http://miniatures.about.com/od/library/printibles/church.pdf
silver city tool shop - 1:87 I think
http://hon3annual.com/silver-city-tool-shop/
Farm silo - N scale I think
https://web.archive.org/web/20000608103146/http://euphoria.benjerry.com:80/fun/village/silo-color.gif
Auto-electrics shop 1:64 on an A4 sheet (I think), not A3 as page suggests
https://papermau.blogspot.com/2017/02/marino-auto-electrics-low-relief-paper.html
Chelsea Hotel Manhattan, New York as a zip file - Not sure about the scale.
https://www.deviantart.com/aeon2/art/Hotel-Chelsea-Papercraft-126017081
Ohio House Hotel, Chicago. Ho
https://web.archive.org/web/20031212144338/http://www.wurlington-bros.com/BuildYour/OhioHouse.pdf
Grain Elevator in HO scale
https://web.archive.org/web/*/billjones_9.tripod.com/* - sort by MINE and download pdf files (or use the filter). Do the same for jpeg files for images
Dale's house - Michigan, USA - Zip files (model and prototype photos) linked from page below.
https://web.archive.org/web/20180207120648/http://members.chello.nl/a.wijers/paperbuildings/download.htm
NOTE - Grain Elevator in HO scale links on this page don't work. See 'billjones_9' above to download that model
General Lee's HQ - 1:160 (not the best quality - looks like a poor resolution photo scan)
https://web.archive.org/web/20070222100549/http://home.stny.rr.com:80/djackso1/papermodels.html
General Meade's HQ link can be found on the same page
Large american Gothic style house. You might want to edit out the logo from this download.
http://www.haunteddimensions.raykeim.com/index380.html
American Gothic style building
http://ravensblight.com/StarcManor.html
'The Olive', a simple US style house - not very detailed
https://web.archive.org/web/20030609235311/http://paperville.com/
Santa Fe lineside huts
https://web.archive.org/web/20031206014600/http://atsfrr.com:80/resources/MillerJay/index.htm
Small library, San Angelo, Texas
//web.archive.org/web/20030407031431/http://www.llrichardsonandfriends.net:80/library/smalllibrary2.pdf
https://web.archive.org/web/20021223125529/http://www.llrichardsonandfriends.net:80/library/smalllibrarytips.PDF
Minnesota Museum of the Mississippi
https://web.archive.org/web/20031214223227/http://www.wurlington-bros.com/BuildYour/build.pdf
A simple rural gas station.
https://papermau.blogspot.com/2014/08/speed-queen-gas-station-paper-model-by.html
Violet Ray Gasoline Station, 799 Alma Street, Palo Alto, Santa Clara County, CA 1:87
http://gtp.de/kartonmodellbau/violet-ray-gasoline-station-usa.htm
Horace Allen Gasoline Station and Garage, San Jose, California, USA
http://www.gtp.de/kartonmodellbau/horace-allen-usa.htm
Whitewash city Imperial Saloon 1:60
https://web.archive.org/web/20060615061603/http://erichotz.com/wwc_free1.html
Byrd School, Oregon, USA - 1:160
http://web.archive.org/web/20030526103253fw_/http://www.peterjvisser.demon.nl:80/models/byrd.pdf
Chamberlain Meat Market & Ray`s Eletronics Building 1:64
https://papermau.blogspot.com/2018/01/chamberlain-meat-market-rays-eletronics_2.html
Stein Building, Illinois
https://krafttrains.com/Paper_Struchers_for_Trains/HO/Coffee_Shops_&_Restaurants-(HO-Buildings)/Building-3-(Cafe-Shop)-(HO)/Coffee_Shop-(HO_Scale.htm
Also see https://www2.illinois.gov/dnrhistoric/Preserve/Pages/construct_mainstreet.aspx
General Store 00 Scale
https://web.archive.org/web/20190715193336/http://www.modelbuildings.org/free.html
https://web.archive.org/web/20200402171127/https://s3.amazonaws.com/ModelBuildings/FREE+BUILDING/Free_Model_Railroad_Building_Sample.zip
Built-Rite Downtown Grocery Building
http://toyconnect.blogspot.co.uk/2015/07/bj-george-built-rite-downtown-grocery.html
Kellogg's Fire House - N Scale
http://toyconnect.blogspot.com/2015/11/kelloggs-engine-house-to-fire-house.html
Joe`s Cafe - 1:64
http://dioramau.blogspot.com/2013/10/cafe-diorama-papercraft.html
Western Railway Station (scale 1:120) - rar file
https://web.archive.org/web/20181005072251/http://mikromodele.cba.pl/architektura.htm
The https://papermau.blogspot.com/ is worth looking at, in particular https://papermau.blogspot.com/search/label/architecture