There is not a single Library of Congess subject heading "Primary Sources," partly because there is no uniform definition of "primary" for all disciplines. However, the sub-heading "Sources" is frequently used at the end of any subject heading to help identify what are essentially primary sources. Often, the editions cataloged with these subject headings may be modern compilations of historical documents. For example:
The subject heading "Indians of North America--History--Sources" leads to a book such as The Elders Wrote: An Anthology of Early Prose by North American Indians, 1768-1931, ed. Bernd Peyer (Berlin: Reimer, 1982).
Some additional sub-headings that may signal a scholarly edition, compilation, anthology, facsimile, or reprint of a primary source relating to the subject heading are:
ArchiveGrid. ArchiveGrid is a collection of over two million archival material descriptions, including MARC records from WorldCat and finding aids harvested from the web. The simple "search" box almost gets lost -- it's in the upper right corner of this homepage.
WorldCat. The largest bibliographic database in the world, since virtually all North American research libraries and many in Europe, Australia, and other continents use it for their shared cataloging records. For searching for manucript/archival materials, check the box "Archival Materials" in the "Limit Search" area, then search for keywords, subjects, etc. as you would with the database as a whole.
Online Archive of California.OAC identifies and provides access to materials such as manuscripts, photographs, and works of art held in libraries, museums, archives, and other institutions throughout the state of California. Includes detailed finding aids; also images and many digitized texts.