As you are already aware, there is a copyright issue involved with the vast majority of film, and only those in the 'Public Domain' are free of such restrictions. To determine the status of any product that might be old enough to be in the Public Domain, contact the Copyright Clearance Office of the Library of Congress by mail or via their site:
www.loc.gov Note that even if an item is no longer under active copyright, the source you obtain it from may have restrictions in their contract which can act almost like a copyright, and you would be legally bound to follow their wishes. As the Copyright people will tell you, however, many films are registered with provisions for non-commercial use, such as the school situations. The main difference, as the notices on some videos make clear, is that if you charge any type of admission, even 'donations,' you fall under many restrictions. Still, to just show any film without charging or accepting any thing of value in return does not automatically allow one to use someone's copyrighted product without permission; that is what the Copyright Clearance Office is for. In the video store's case, they either got clearance from the CCO, or they simply took the risk that no one would report them since they do not do it all the time. Check you local libraries or
www.Amazon.com to find the books on copyright. The National Assoc. of Theatre Owners can probably direct you to any number of distributors/bookers who supply the films commercially. This site lists many distributors, primarily for IMAX, but they no doubt also carry standard 35mm and possibly 16mm also:
http://www.1570films.com/distributors.htm This subject has been discussed before on these Forums, so you may want to check the Archived posts for more.