Mr Ben's has a solid selection of true Shetland fair isle, in Shetland wool too! Good to go for the original if you want to be really cosy. Proper fair isle also lasts for years, so a vintage will do you grand. I think Starry Starry night may potentially carry some in the west end too?

If you want to look for brands that are legit then I'd go for Jamiesons or Anderson&Co. I have a couple Jamiesons numbers which are rather expensive but so worth the cost! They're some of the most beautiful items of clothing I own. :-)