Have try of the bike shop in Knutsfords the one one the end of the main shopping street. been in there before and seen a load of marin forks being sold as singles. No idea what sizes they are or vintage.
I have some nos chromed threaded ones that were taken off a new Marin Indian way back in 1995 and were put in my loft. There they still remain unused if anyone is interested? Logos and forks are mint.