My local Jewel carries Morton Rock Salt off and on but mostly seasonally. Look for a red labeled Morton box. It would be worth calling around to a bunch of Jewel and Dominicks stores. At my Jewel, it is on the shelf next to the pickling salt (green label) and large boxes of table salt (blue label). Look waaaay down on the bottom shelf.
I've purchased bags labeled for ice cream and found them to contain gravel bits. Since the salt doesn't come in contact with your ice cream mix, you could use snow melting salt but be certain it is just salt.
"The only thing I have to eat is Yoo-hoo and Cocoa puffs so if you want anything else, you have to bring it with you."