Contact the playwright. Less well-known plays produced by small theaters or amateur production companies, such as those staged in schools and hospitals, are usually not available from bookstores or theater databases, particularly if a play has been recently staged. If the script is for your own private enjoyment, the playwright may be flattered and happy to give you a copy, although you need her explicit consent and may have to pay royalties if you want to stage the play yourself. As the author's intellectual property, the play is protected by copyright laws.
Look in theatrical book stores. Such stores specialize in theatrical scripts and books about the theater. Stageplays.com offers a large collection of plays and musicals for all types of dramatic productions, including royalty-free ones. Large book sellers, including online ones such as Amazon, also sell scripts of well-known plays in traditional book form, along with additional memorabilia, such as souvenir theater programs. These scripts include special editions for actors, designed to make it easier for professionals to interpret and stage the play. Celebrated plays, such as Mary Chase's "Harvey," may also be downloaded in digital form from online booksellers.
Consult a database. Theatrical databases simplify the process of locating a script and provide links to sellers and publishers. Visit FindaPlay.com and simply key in the name of the play you are looking for. Alternatively, try playdatabase.com, which offers a comprehensive selection of scripts and monologues.
If the play is long out of copyright, try Project Gutenberg (www.gutenberg.org), where texts are free to download.