Despite what others may tell you, it's not legal to play a film in public even if you own a DVD. If you're a library, then you can license a lot of films (including many in my collection), from Movie Licensing USA. Otherwise, we can clear rights through the studios or Swank, depending on the title.
If you are a theatrical exhibitor, then we need to contact either your booker or the owning studio. (You are probably not a theatrical exhibitor unless you own a theater that can advertise in the newspapers!)
Doing legal showings is important to me. It's important for you, too. You don't want to be sued any more than I do! Besides, we want to pay the owners just because it's the right thing to do.
I realize that in some cases that paying copyright royalties is a deal-breaker. It just makes the budget go too high. The good news is that I also maintain a collection of public domain films. Public domain films have expired or non-existent copyrights. These can be run without royalty payments. If you want to run a well-known title, then it's probably copyrighted. If you're willing to settle for something lesser-known, often still with big stars, then I can probably help you save money.
Unfortunately, there is not yet a comprehensive database of copyright owners. The Library of Congress has a partial one, and there are copyright renewals listed in a set of expensive books. I can help you research the rights on a particular film if you need. In the future, I hope to compile enough public information to make a database that helps bridge the gaps in the online databases. Some of the extant resources can be found in my Links section, but these are not comprehensive by any means!
The maze of copyrights, renewals, differing owners and other things can be confusing, especially if you're new to this. Let me help you. I do this every day.