As someone who collects Playbills, I can try and help you out!
First, I would address your envelope to the "Stage Manager of _____________" In the letter I would try to mention something about the show, or something about the cast. If you've never seen the show, but plan to, mention that. If it's really difficult for you to actually go and see the show, give a reason why you can't see it. Try and mention something that you like, either the sound-track or something about one of the actors/actresses.
You do need to send a SASE (2 US Forever stamps if you're in the USA will cover the postage for one Playbill).
Hope that helps.
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