Library of Congress Subject Headings are quite useful for discovering primary sources. The following subheadings usually are added to indicate that the material is a primary source: sources, personal narratives, correspondence, diaries, manuscripts, "early works to 1800" or notebooks. Once you've discovered the subject heading for secondary sources, try adding one of the primary source subheadings to see what you find. 

Secondary source subject heading: France -- History -- Revolution, 1789 1799

Primary source subject heading: France -- History -- Revolution, 1789-1799 -- Sources

Anthologies of primary sources, typically edited and annotated by scholars, offer a useful avenue for perusing primary source materials. Here are a few selections, available for check out at Collins Library.

Collins Library provides access to the following subscription collections:

Many more primary source collections are available freely online, most of them digitized and curated by academic libraries.