Add 'sources' to your keyword search in OhioLINK. The subject term for many primary documents uses the word 'sources'. You can also try words like 'letters', 'correspondance, ' 'personal narratives', 'diaries', 'papers', 'journals' or 'oral history'.
Many museums, libraries, universities and other organizations are digitizing primary sources for scholarly use. Try a google search using 'primary sources' as one of your keywords, or other words such as letters, etc.
For best results, limit your search to sites hosted by educational institutions (.edu) or the government (.gov). Do this by using Google's advanced search, or just add 'site:.edu' or 'site:.gov' to your search.
Be careful with online sources - check their 'about us' page to see who they are. Look for citations of the original source.