I was able to buy the bath salts cheaply in a big bulk bag intended for spa flotation tanks. I had a huge bag which I slowly used over the year.

You'd have to look around on the internet for a supplier.

For longterm storage don't store the bath salts in the bathroom, they soak up the moisture in the air and do not store well for as long.

You increase absorption if you use a body brush to brush all the dead skin off before the bath. Good to shower it all off afterwards. Don't soak to hot or more than 40 minutes.