Just send it to the actor c/o the theatre. If you are asking for something to be returned, be sure to include a SASE...or you will probably never hear from them.
You can google the address of any theater.
And while you can ask you cast member to pass along a playbill for signing...you will probably get a better response with asking individually.
If we're not having fun, then why are we doing it?
These are DISCUSSION boards, not mutual admiration boards. Discussion only occurs when we are willing to hear what others are thinking, regardless of whether it is alignment to our own thoughts.