Browse a large collection of thematic and historical maps from around the world.
Bibliotheque National de France "Gallica". Like the Library of Congress and many other national libraries, this institution archives and displays digitally many historic maps.
Altea Map company - https://alteagallery.com/ - a dealer of historic maps in London. Although these are all for sale, scans are large and downloadable. Given there inventory there's likely something you can find for your project. Don't forget, if you find a map, download it for yourself--you never know when it could be sold.
Other Specialty Maps
"Redlining" maps (Home Owners Loan Corporation) Mapping Inequality Project HERE - maps showing the racial (and other) descrimination in the housing industry during Jim Crow.
Sanborn Map Collection (Library of Congress) - fire insurance maps of the early 20th century. In many cases these are the only accurate know maps of a city on a house-by-house basis. [open access] HERE