Yes, Thorn Is SJS brand, Stewart Thorne is the guy that owns the shop, I think.
they are the Ones that contracted to have them made, So No.. get them shipped From England.

But its a simple item for a US/CDN welding/machine shop, commercial or home based, to make..

what handle bar bag specifically are you working with? have a picture of the back side, where it is mounted? its mounting scheme..
Straps based .. consider fittings that are made to mount 2 water-bottles behind the saddle as a way you can fit a bag on them ..

I was able to use one to fit my carradice bag to a plastic saddle.. with one Minoura made..