Look at the catalog record for a primary source or sourcebook on your syllabus or on Course Reserves. Click on the Subject Headings to find more titles with the same subject heading.
Watch for these words in the Subject Headings - they are Library of Congress Subject Heading controlled vocabulary indicating primary sources
sources, letters, interviews, speeches, personal narratives, diaries, correspondence, sermons, notebooks, sketches, description and travel, treaties, pamphlets, biography (includes memoirs), newspapers, periodicals, pictorial works, art, architecture, portraits, caricatures and cartoons, cookery, decorative arts, furniture, material culture, guide books, maps, fiction, poetry, periodicals, newspapers, bibliography, early works to 1800
Use them in keyword searches in the library catalog: example (pope or papacy) AND (sources or correspondence)
This is one research technique, but not a perfect system. Most topics fall under multiple Subject Headings. Many wonderful sources at Vassar do not have Subject Headings indicating they are primary sources.