Hmm...had no luck with the websites. The ones that carried McNeil of Barra didn't carry it in synthetic...and those that carried synthetic tartan didn't offer it in Mcneil.
I strolled through my local Joann's today. I'm in that place all the time since I love to sew, but I didn't find anything there. They had a few tartan-like fabrics, but when I asked if she could order me some "McNeil of Barra", the girl looked at me as though I came from Mars. So no luck there. But I'm going to keep on searching.