If one is trying to buy cheap chemicals you might wish to find a chemical supply house. Look under "Chemicals" in your yellow pages.
I added this to a post but I thought it might be helpful to others in lowering the cost of supplies. The idea was given to me by a friend who is a geneticist who thought I should give it a try, and indeed it did help.
The supply houses usually supply chemicals to labs, hospitals, the metal industry, hotels, etc.
Where I live 15 lbs of Calcium from the pool store costs $32!!!! That's Hawaii for you. From the local chemical supply house it costs $24 for 50 lbs. Most of them carry Muriatic Acid (4 in a case), 22% cheaper than a building supply store, and 45% cheaper than the pool store, and they also carry Cyanuric Acid. but not Borax.
I did not ask about chlorine, as we have an SWG, but I believe they might also carry it, as they supply hotels. I should have asked about salt but I did not need any at that moment.
Usually there is a minimum $ amount, in our case $50.
Hope this helps.