Get a bag of the water softener salt, it's more expensive though, then get a big hammer and pulverize it until it's about the same size as the inexpensive table salt. When you're through doing that, get a 5 gallon bucket from Home Depot to store the unused salt in. Make sure it's water tight as any moisture will most likely make the salt unusable. If you get the big bag, you might need two buckets. This would be a great project if you have the grandkids visiting...:W
Personally, I don't mind paying the $3 for the 2 1/2 pound box at Sam's Club. Good for three flushes and it's always fresh.....Dennis