19th Century U.S.
20th Century U.S.
Key Sources - All Eras
HathiTrust contains millions of digital books, journals, government documents, and other volumes, all digitized from research libraries. The collection includes both public domain and in-copyright works across a full range of subjects.
Find a good secondary source, and work backwards. See what primary sources the author consulted, and search for them (you will probably find other relevant sources this way).
Think like an archivist. In archives, sources aren't organized by subject as they are in the library, but by the person, organization or group that created them. Start from what you know -- names of people, organizations, schools, or programs -- and do an internet search using that name plus a word like "papers" or "collection." Example: the Mary Lyon Collection
Tips for finding primary sources from the Library at UC-Berkeley - Includes information about finding particular types of sources and searching with information you already have.