This is probably already known here but just in case.

I bought Pure Magnesium Chloride Brine Flakes.

1/2 cup water in a saucepan and bring to a boil
add 1/2 cup magnesium flakes

It will dissolve in less than a minute or so. Then remove from the heat, cool and put it into a little spray bottle. Much less expensive than buying the oil and why when it's this simple to make your own?