I don't think anyone local sells it any time of the year. I had to order some off the Internet. It lasts much longer than you expect and. Went with the pedal ow driveway heat from CVS.com. I know that seems weird, but CVs was the cheapest.
Actually from my experience a few years back calcium chloride is much easier to get the magnesium chloride.
It is used to harden concrete especially at lower temperatures as well as ice melt for drivewarys/sidewalks.
You might try calling around to local redimix companies and ask if they have calcium chloride.
years ago sherman had cal chlor in 25 pound bags for like $8 or so. (that is the anhydrous form that is 92-96% pure).
Another redimix had the old dow flake in 50 pound bags for $20 or so.
I found calcium chloride and magnesium chloride at Brentag which is an industiral chemical supplier here in huntsville.
They normally did not stock magnesium chlorid but I could order a 50 pound bag and they would not charge for shipping. So it was $30 for a 50 pound bag.
I think up in the attic above the garage I may have whatever remains of either which you are welcome to a few pounds of.
But it has been a few years since I checked that out. LOL