I have a few from Harley of Scotland, which can be purchased at Unionmade and Bahle's (no experience buying from the latter store). And I have one from Jamieson's, which are available at End Clothing and No Man Walks Alone. Both are nice. I prefer Harley because the fit works better for me but YMMV.
I've heard people mention Spirit of Shetland as well but couldn't access their website a few minutes ago, so I'm not positive if they are still around.