I hear Manitoba has plenty of them...
Around here you're probably going to have a hell of a time. I tried buying sidewalk melty salt a month ago and hardware stores and gas stations laughed at me. Seasonal, y'know?
Two options I can think of that might help are
a) find your own sacks and fill them with gravel
b) take the four that I pulled out of the back of my truck and left in my uncle's garage (free!)