If you want to look at the library's physical collections, here are a few tips:
The Library of Congress Classification System groups plays by country/language, time period, and author name. There are also special sections within these that are specifically for collections of plays. Use this link to help narrow your possibilities.
Searching for a specific playwright or specific play?
No specific play/playwright in mind?
If you're looking for shorter plays, try searching the catalog for one-act, ten minute, or short plays. Also, the Humana Festival publishes the complete plays from each year and there are some short plays mixed in, but they are not grouped together.