Robin, they were fitted to early Ford Transits, so thats one source. Another is generators as they were used for that sort of application for a while.
They crack heads so make sure that whatever you get is OK.
Robin, don't know what area you are in, but there was a boatyard in Oulton Broad near Lowestoft who was replacing the engines in its hire fleet or replacing its hire fleet, and reportedly had a few Perkins not sitting in its workshop whether or not they were 4108's or 4107's I am not sure. Topcraft Marine. Maybe can help.
"I am the master of my fate: I am the captain of my soul."..........I think????