I remember this from my youth; it was not uncommon for local bakeries to do a nice SRB, and see strong local sales. But hardly anyone makes this for sale anymore.
No salt actually used; natural bacterial fermentation, on scalded milk and corn meal, then flour and such to complete.
Fermentation temp was controlled by heating salt, and placing the starter in it to maintain temp.
Breadsmith says they make it and there is one close, although I think they use a commercial starter. No one else within about 70 miles.