I have been using DermaMag from Magnesium Direct with excellent results. Evidently, DermaMag also comes from the Zechstein seabed and may be the purest magnesium oil yet.
It is the same exact authentic magnesium oil that Dr. Mark Sircus talks about and even comes with the "Zechstein Inside" logo on the bottle.
The nice thing about it is that it comes in the 32-ounce size so you dont have to make a $100+ purchase to get enough for the footbaths.
Their website also has great info on magnesium chloride in general.