Primary sources are "first-hand" information, sources as close as possible to the origin of the information or idea under study.
In contrast, secondary sources, works that provide analysis, commentary, or criticism on the primary source.
Primary sources include original creative works (movies, television and radio shows, advertisement), books and other written works by people who were participants or direct witnesses to an event (memoirs, personal correspondence, oral histories, and contemporary newspaper accounts).
The Online Archive of California provides free public access to detailed descriptions of primary resource collections maintained by more than 200 contributing institutions including libraries, special collections, archives, historical societies, and museums throughout California and collections maintained by the 10 University of California (UC) campuses.
SNAC (Social Networks and Archival Context) is a free, online resource that helps users discover biographical and historical information about persons, families, and organizations that created or are documented in historical resources (primary source documents) and their connections to one another. Users can locate archival collections and related resources held at cultural heritage institutions around the world. Keep in mind that although SNAC is working to build a corpus of reliable descriptions of people, families, and organizations that link to and provide a contextual understanding of historical records it still only includes a small number of members which can be found here and shouldn't be your final stop.
Becoming familiar with professional organizations involved in preserving, providing access to, and engaging with radio & television collections can help you learn about the state of the field and ongoing conversations, member archives and their collections, recent acquisitions, and new scholarship and other projects.