Some supermarkets do but only in areas with a sizable jewish community.
All supermarkets do a certain degree of specialist products if the area demands it.
Unfortunatly the supermarkets around me dont carry anything I ever need for any interesting cooking, so have to go calling on the specialist food shops instead.
You should be able to get kosher salt in a cash and carry type place or cooks shop however, as its used a lot in professional kitchens as well.