A primary source is a document that was created during the time period being studied. This includes newspaper and magazine articles, original film reviews, pamphlets, interviews, government publications, manuscripts, diaries, and other sources that speak to the context of the time period. Primary sources provide firsthand evidence of historical events recorded by those who lived it, and as such usually need to be contextualized with more recent secondary sources like histories of the time period and academic film criticism in journals.
You can usually find clues to specific primary sources by looking through secondary materials like books and scholarly articles written in the present day. These often include bibliographies of the primary works used as references, and can give you ideas about other ways to search for your topic. Some more recent books also include reprints of primary sources.
Both the "Newspapers and Reviews" and "Journal and Magazine Articles" tabs above contain links or citations to historical primary sources and secondary sources.