Something that surprises folks who buy plays and screenplays off of Amazon or Ebay is that they can't actually perform the play in public. The written script is usually sold by the authors so potential directors and producers can read all about it. 

After that is accomplished, and if the director or producer wishes to commit the script to screen or stage, they have to purchase performance rights, otherwise known as dramatic rights.

Institutions such as schools and universities are less prone to just taking someone else's work and staging it, even if it is for charitable purposes. Small theater groups are usually the ones who try to fly under the radar, but only if their leader is not on the up-and-up. Most are, so in the end, dramatic plagiarism is relatively rare.

If a script is available for purchase, such as Roy C. Boothe's, How to Make a Brain Souffle, it usually has the performance contact information right in the book. Once a license is purchased, the theater or cinematographer can proceed. Some authors give huge discounts for works that benefit charities, so make sure you let them know when negotiating performance or dramatic rights.