Leverage the work of librarians who have created digital user guides like this one for finding primary sources online. Use Google or your favorite search engine to try this search, and explore the results you see:
primary sources libguides
Digitized page images of historic American periodicals published between 1684 and 1912 on advertising, health, women's issues, science, the history of slavery, industry and professions, religious issues, culture and the arts, and more.