www.schoolhousepress.com is where I've gotten mine. It's a fingering weight wool, made for traditional Fair Isle sweaters. Several books that I have use it.
I've also made Fair Isles out of dk and I love them. I wanted to make a pattern that called for the fingering weight and I just made a smaller size. Much to my amazement, it fits.
I don't see why using the colors of the tweed as accents wouldn't be a good idea. :shrug: I like it, actually.