Read the introduction on "public domain & copyright friendly licences" to learn more about them.
In Google Images, there is a "usage rights" filter to find images that can be copied and reused. Many of these images are licensed under Creative Commons or in the public domain. However, this search filter is not foulproof; some "All Rights Reserved" images may slip through in the results. Be sure to go to the image's source page to check the Terms of Use or licence type to ensure it permits your use.
Please refer to the section on General Photography, Creative Common & Public Domain of the Finding Images Guide.
Public domain films are not under copyright protection and can be copied, used and incorporated into other works.
Below is a selected list of websites with information on public domain films. You can find similar websites by doing a Google search. In some cases, the films (e.g. Internet Archive) can be freely watched and downloaded online. But in other cases, they are commercial sites that charge a fee for sending you a copy and/or restoring an old film to a usable format. The reliability of public domain information varies on these sites so it is recommended that you do some research to verify the copyright status if you want to adapt and/or incorporate a film clip into your own work.