Magnesium Chloride, dead sea salts, available fin 25kg bags from suppliers of agricultural/Equestrian Products/farm supplies CWG is my local place but there are plenty of others Countrywide are a UK chain that may also sell it.

I pay around £6.00 a bag and chuck a jugful in the bathwater and soak for 20minutes. There is evidence to show the magnesium is absorbed transdermally. More information here