In OneSearch, try searching with the phrase “primary sources” in the All Fields or Subject Begins With… search boxes. In addition, think about the type of source and search using those words, e.g. - correspondence, letters, diaries, narratives, and speeches. For example, if you are looking for primary sources related to abolitionist Frederick Douglass, try searching in OneSearch for: speeches and douglass
Some databases, such as Academic Search Complete (and other Ebsco databases) and Opposing Viewpoints in Context, will allow you to select Primary Source as a document type when searching. In Ebsco, look for Publication Type on the main search pages and choose Primary Source Document from the drop-down list. In Opposing Viewpoints in Context, choose Primary Sources from the list on the Advanced Search page, or run a search and look for the Primary Sources results section.
WorldCat (the online catalogs of 10,000+ libraries worldwide) is another good place to look. From the main search page, choose Advanced Search from the Everything tab. On the next page, type your keyword(s) into the first search box, then go to the Format options and choose Archival Material before selecting Search.