Any good submarine shop....Blimpy, Subway, Quiznos...
Sorry I just hadta.
Or call Matthey McConawhatever out there in LaLa land. I think I heard him say he liked to trick up his Airstreams with nautical windows.
Real Portholes are available from some yacht suppliers but not sure if the thicknesses are compatible. Have you googled portholes for sale?