Very few screenplays are published commercially. Screenplays that are published tend to be either those recognized as film classics (e.g., Citizen Kane) or those written by well known authors (e.g., Quentin Tarantino.)
Start off by doing an advanced search by title:
[Screenshot of the USC Libraries Advanced Search box]
If USC owns a copy of the published screenplay, it will almost always be found in the Cinematic Arts Library. If you do not find a copy of the screenplay in the Libraries Catalog, this means either USC does not own a copy of the published script--or more much likely--the screenplay was never published in the first place.
The Cinematic Arts Library has a large collection of unpublished screenplays. These are available for limited circulation to current USC students majoring in the School of Cinematic Arts or USC students currently enrolled in screen-writing courses through the School of Cinematic Arts. For more information, please inquire with cin@usc.edu