Does anyone know where I can find an English translation, preferably online or in a book the library of a small liberal arts college would be likely to have, of the original proof of Pappus' hexagon theorem from projective geometry?
This is available in the Loeb Classics Library, Greek Mathematical Works, Vol II (edited by Ivor Thomas), page 600.The Loeb CL can probably be found in its entirety in many liberal arts college libraries. For your convenience, you can find the book here.
Stumbled by coincidence today over the book "Sir Thomas Heath: A History of Greek Mathematics, II". Chapter XIX (pp. 355-439) is devoted to Pappus and contains a lot of (mathematical) stuff. Perhaps you can find there what you are looking for.
You can find the book at the Internet Archive (PDF link on the left, download may take a while).