The New York Public Library has put an amazing collection of hi-res Open Access Maps online for downloading. There are over 20,000 cartographic properties which they believe have no known US copyright restrictions. Because of that, they have put them all under a Creative Commons CC0 1.0 Universal Public Domain Dedication. You do have to create an account, but that is free within the bounds of being asked to support the library every so often. There is a bit of a learning curve finding your way around the site, but it doesn’t take long to pick up on it.
