To start, try ArchiveGrid, but you can also try WorldCat and Google.
Trace back from secondary sources: what sources are other scholars using to analyze and provide evidence for their arguments? Are these the types of materials that you are interested in utilizing?
Talk with an archivist! Archivists have knowledge about the landscape of institutional collecting and can point you in a good direction! You can start with the archivists here at Barnard! archives@barnard.edu'
Check out this Research Guide for great and comprehensive information on working with Archives.
Some of these resources represent digitized archival materials from specific institutions; others are portals to multiple institutions' digital collections or databases collocating digital archival materials on a given topic. These collections include some formats highlighted elsewhere in this guide, including newspapers, government documents, and books; in addition, they contain manuscript materials (e.g. letters, notes, drafts of publications), photographs, and more. Archives are rarely able to digitize the entirety of their holdings or the entirety of a given collection; you should look to see full descriptions of collections on an institutions' website to get more context for digitized archival materials.
Search the CLIO Catalog to find primary sources by adding keywords to your search, or look for them in subject subheadings or in the Subject - Genre filter. Here are some examples:
CLIO is the library catalog for the Columbia Library System (including Barnard Library, but excluding Teachers College). In CLIO you can find books and media materials, search for different kinds of databases, and find articles within those databases as well.