Solar salt from the big box hardware stores. Get the stuff in the blue bags. It's $4-something for 40 lbs.
An excerpt from another thread:
"The best thing to use to raise the salt level is water softener salt. Look for salt that is 99.4% pure or better and doesn't have any rust inhibitor or other additives. The best choices include Diamond Crystal® Solar Salt Extra Coarse Crystals in blue bags, Morton® White Crystal® Water Softener Salt in blue bags, or Diamond Crystal® Sun Gems® Crystals Water Softener Salt in yellow bags.
Water softener salt pellets will also work, even though they dissolve more slowly they are still fine. Pool salt is also fine to use, but tends to be much more expensive. Potassium chloride will also work, but you need 17% more of it and it is more expensive. Avoid table salt, rock salt, and deicing salt."