Copyright is a form of protection provided by the Congress of the United States to the authors of "original works of authorship," including literary, dramatic, musical, artistic, and certain other intellectual works. This protection is available to both published and unpublished works.
See UK's Copyright Resource Center
Listed below are some different websites that are great places to turn to when looking for various media. These sites have content that are either in the public domain or under a creative commons license. For any resource you use, make sure you look at the permissions so that you are in compliance with legal and ethical measures.
Instructions for how to search on Creative Commons:
A public domain work is a creative work that is not protected by copyright and which may be freely used by everyone. The reasons that the work is not protected include:
(1) the term of copyright for the work has expired
(2) the author failed to satisfy statutory formalities to perfect the copyright, or
(3) the work is a work of the U.S. Government.