The covers are out there, found
this, (scroll down to just past the halfway point). Unfortunately no mention of the seat cover vendor. Of course it could have been a shop sewn cover.
Well, after more netsearch, I'm thinking two options exist:
1. Buy from the vendor of choice, then take the cover set to an upholstery shop and have them cut/sew as required and add piping so it matches the original style.
2. Source original style material and have a shop make proper covers using available photos for reference.
Good Luck and do post up if you find a solution.
RonR