Primary sources are found in many kinds of organizations according to their missions and the areas in which they collect. Many documents are only available in physical formats making them hard to access, but even when they've been digitized they can be hard to find because they're often locally hosted rather than aggregated, centralized, and made available through a single interface––like our multidisciplinary databases, for example.
For this reason, you can often discover primary sources by mastering the ancient art of Google Fu. Pair keywords derived from your topic, especially exact phrases (e.g., "white minority rule"), with words such as
Or search your topic keywords + "LibGuides" to see what other librarians have collected.
Subject headings constitute a controlled vocabulary that can help you find sources by their primary subjects. The Library of Congress maintains this vocabulary making it standard across many different databases and catalogs.
For example, imagine you're developing a topic on theme parks. Searching that term in LC's subject heading thesaurus reveals the preferred subject heading, "amusement parks." Using the advanced search interface in a given database, you can change the search field from keyword to subject as pictured below. Searching "amusement parks" in the subject field returns all results tagged with that heading. Now cross-reference your subject search with the word "sources" to locate edited or republished collections of primary sources.
Caveat searcher! Constructing a query like this may filter relevant results. Begin with broad, keyword-based searches, then search by subject when you have a well defined sense of the resources you need. Research is often a process of moving between broad and narrow levels of inquiry.