I get mine from wallmart
If you go looking though enough of the ice melts you can usually find one that is just a bucket of mag-chloride and nothnig else (except probably a little contaminate) but I have been using it for about 1 year now without trouble. and 20lbs is about $6.
I also hit the pool place for my dowflake. It's used to increase calcium hardness in pools. Somewhere about $4-$6 per lbs.
I can always find calcium chloride in the driveway-melt. But they always add urea and other colorants
