I'm with Slohairt. Stobie is a familial form of Stobo and that's from the barony of Stobo in Peebleshire. But, of course, who knows where your ancestor lived post 14C? Perhaps on some Stewart estate elsewhere in the counry? That's very likely. Which Stewart tartan did your grandmother point to when she said that was her family tartan? There are a whole bunch of them -- red to green to blue to black -- but if you can find the one she said was her's we will have a whole bunch more info to work with.

In the meanwhile, to keep your interest alive, George Black wrote that Major Robt Stobo was guide for the Fraser Highlanders in their climb to Abraham in Quebec in 1759. Sneeky devil! Knew his way about in the dark, that man.