I get Dow Flake and Magflake down in SW Florida at tennis supply that caters to tennis court operators. They sell it to keep dust down on clay tennis courts. It costs a buck or two more, but it beats shipping it in from up north!
BTW, I've been using Dow Flake (calcium chloride), Magflake (magnesium chloride), soda ash from a pool supply (sodium carbonate: alk) and Epsom Salt (magnesium sulfate) from a California bulk supplier for about 6 or 7 years now. I probably pay about 20% of what I would pay for the best price at Bulk Reef Supply.