One of the best ways to search for primary sources is to use SuperSearch. Type in your search terms, then use the limiters on the left-hand side of the screen to narrow your search to primary documents and by date:
Enter dates below the content type to find materials from the historical period you're interested in:
Books
Archival Materials
Archival Collections
Books
Newspapers and Periodicals
The West, Manifest Destiny, and Expansion
Newspapers
General Sources
Legal Documents
Newspapers and Media
Radicalism, and Activism