You can add "motion picture" or "motion pictures" as a keyword or subject term when searching...
However, it's hard sometimes to guess the other words that might be used in the book, so use this judiciously.
Film sources are largely in the 790-792 section (entertainment); you may also find film sources in the 800's (when considered as "literature" of a certain culture), the 300's (if the primary topic is sociology), and other areas.
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