Go into a rural area that has a grain elevator and you can buy salt by the 50 pound sack or more...cheap also....
You can also pick up salt blocks at your local feed store pretty cheap in 4 and 50lb blocks. I put back a few 50lb blocks in mylar bags just because they are so easy to store away and take up less space. I keep a block out year around for the girls anyway. I’m also planning to add a goat or two and rabitts this next Spring so I figure I’ll use it up eventually.