If you live in NYC you could try the NYPL library near Lincoln Center, if you know anybody that goes to AMDA they may be able to get it from their library - or if you know anybody that ocasionally dabbles in broadway scores you could just talk to them and if they dont have it they could probably get it.
The most legal thing to do is find information to write to William Finn or his publishers, and get it from them. Most composers are very kind hearted when it comes to education
You learn to play the straight man, the lines become routine - never really saying what you mean - but i know the scene will change :)