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Out here in the West, I'm not sure one can buy rock salt like used up
Nawth. At least in any large quantity.
I've seen it here in the Sowth but I can't remember where or why. It
does get icy every few years but everyone just sleeps in.
Must be a hardware store. Rock salt is used for making ice cream in
the Sowff. It's not a miff, er, myth.
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Everyone throughout the USA can buy rock salt locally, but not the kind typically used for melting snow.
Rock salt is used in water softeners, so, anyone living in a rural area that gets their water from a well and isn't supplied with soft water from a nearby city or town will either have to live with hard water or have their own small water softener in their house for their own use.
But the size of the of salt will be different for use in a water softener and for melting ice on sidewalks and stairs. The salt used in water softeners will typically be "1 inch clean", which means salt that'll pass through a sifting screen with 1 1/4 inch gaps between the wires, but not through a sifting screen with 1 inch gaps between the wires. You need large gaps between the salt stones to allow water to flow easily through the salt bed and to have a large area of contact between the salt and the hard water.
The kind of salt we buy up here to melt ice will typically be "quarter inch down", which means everything that will pass through a sifting screen with 1/4 inch wide gaps between the wires. So, what you get in a bag of ice melting salt will be smaller than 1/4 inch in any dimension, and much of it will be the size of sand or even as fine as dust.
The primary difference between the two kinds of salt is that the stuff used in water softeners has got to be food grade salt, whereas the stuff you scatter on your steps doesn't have to be food grade, and while much of it will be white, you'll find all kinds of other garbage in it. Water softener salt will be white as Manitoba snow cuz it's pure NaCl, just like table salt.