To find primary sources specifically, couple the topic of your subject heading with relevant subheadings that relate to the historical time period, place of origin, and item format of the materials that interest you. Use the Library Search for materials.
These "complex subjects" (containing more than one idea) have their own particular notation.
Example: if you are looking for letters written by political insurgents, you might try searching for "Insurgency - Politics - Letters" for more targeted results.
Citations and abstracts to hearings, committee reports, bill files, witness testimony, and more issued by Congress from 1769-present. Full-text is available through 2010, later for some titles. Full-Text Hearings 1824-2010; (CRS) Reports 1916-present, committee prints from 1830-current; Serials Set documents from 1789; the Congressional Record (1969-1994 in print); and Executive Orders and Presidential Proclamations, 1789-2017.
Includes multiple databases: International Financial Statistics; Government Finance Statistics; Balance of Trade Statistics; and balance of payment statistics. Data from 1948 - present for some data points.