The public domain is the body of books, plays and other works that are not protected by copyright. It is legal to use public domain works for any purpose, including republishing them and selling them for a profit, without permission from the original author or publisher. Public domain plays may provide you with material to reprint and sell, perhaps as critical editions with commentary, or works to perform without having to pay royalties. In order to avoid violating copyright law, however, you must be completely sure that a play is in the public domain before using it.
Search online databases of plays and other works for plays published before 1923. In the United States, anything published before 1923 is automatically in the public domain. Online archives include The WWW Virtual Library, One-Act-Plays.com, Google Books and The Internet Archive.
Visit your local library (or its website, if it has an online search catalog) and search for plays published before 1923 in the library system. You should be able to search by publication date under "Advanced" in your library's computerized search catalog.
Search for plays published before 1923 in WorldCat, a database of library materials from around the world. Consult a reference librarian at your local library for information about borrowing materials through WorldCat.
Change the dates of your search from before 1923 to 1923 to 1963. This will take a bit more work, since not all material published between 1923 and 1963 is in the public domain.
Search the U.S. Copyright Office's Catalog of Copyright Entries for each play you find published between 1923 and 1963. You can find scans of the Catalog prior to 1978 at Google Books and more recent records at the Copyright Office's website. If the play's copyright was not renewed in the 28th year after publication, it is in the public domain.