Primary sources are anything created during the time period under consideration: published books; unpublished letters; photographs or drawings; music or song lyrics; clothing; objects of everyday life; etc.
Academic libraries and historical museums often try to digitize primary source collections held in their archives and special collections in order to make them available to the widest possible audience.
Google Books contains the full text of millions of nineteenth-century texts. If your research leads you to specific titles, you can search for physical copies in Primo or search Google Books.
The Ngram Viewer lets you explore word and phrase frequency in the corpus of Google Books. It can be useful for detecting trends, but cannot tell you anything about the specific contexts of usage.
Books in Primo are assigned Library of Congress Subject Headings. In many ways, subject headings are a form of tagging, in that they represent the content of the material and provide ways for you to efficiently locate more materials that are conceptually related.
Library of Congress Subject Headings are also quite useful for discovering primary sources. The following subheadings usually are added to indicate that the material is a primary source: sources, personal narratives, correspondence, diaries, manuscripts, or notebooks. Once you've discovered the subject heading for secondary sources, try adding one of the primary source subheadings to see what you find. Here are some examples:
Search the Literature Resource Center by genre to find possibilities!