I think Randy's 3 part uses both Magnesium Chloride AND Magnesium Sulfate.
http://www.reefkeeping.com/issues/2006-02/rhf/index.php#6
4 pounds of epsom salt costs about 3 bucks at Longs Drugstore.
We (forum members) haven't been able to find a good source of local MgCl, though Robert does have an opportunity to get some from his chemical supplier, we're not at all sure about the "other" contents of the MgCl bag. ie; it's not proven to be completely safe. (but what ever is?)
V