Ok here's how it works and my advice. Your best bet would be to write a non-film version yourself, because the story of "The Wizard Of Oz" outside of the movie is public domain. So my advice would be for you to make your own version and it won't cost you anything. Otherwise here is the information:

The only way to legally obtain the rights to a stage production of "The Wizard of Oz" film version is to contact Tams Witmark Music Library in New York City. They are the sole owners to the rights to the 1987 Royal Shakespeare Company version (basically the film script and music) and the 1950's Muny adaptation (which is now extremely outdated and almost never performed anymore).

You must go through Tam's and pay royalties (depending on the size of the theatre) if you want to perform ANY version that features the dialog and/or the songs from the film.

The address for Tams-Witmark is:
Tams-Witmark Music Library
560 Lexington Avenue
New York, NY 10022
212-688-2525
or 800-221-7196