The U.S. Census maps are in the Statistical Atlas of the United States. Quoting from the site:
OF THE NINTH CENSUS (1870) published 1874
Presented here are all of the maps and charts from the first statistical atlas of the US Census, widely praised in its time and still a wonderful example of sophisticated graphics, the out-of-date racial/psychological nomenclature notwithstanding. The atlas is available page-by-page from the Library of Congress, but you can download it in bulk here: download ZIP files:
All 54 Maps and Charts as 96-dpi JPG files (55 MB) — displays at actual size on most screens. All 54 Maps and Charts as 300dpi JPG2000 files (263 MB), identical to the files posted individually by the Library of Congress. The Atlas also included 53 pages of scholarly discussions of the population and geography of the country. The articles are mainly of historical interest, but you can download all 53 text pages as 300dpi JPG2000 files (253 MB) if you'd like, or go to the Library of Congress for individual pages. Unfortunately it's not worth downloading mid-res versions: at lower resolutions they're very difficult to read.
You may wish to explore some of the other fabulous maps on this site.