Why not ask your co-op if they can get it with thier stuff, freight for them is prolly WAYYYYY less than for you since they have a place of business. You may even be able to barter some of your bath stuff.
I also use epsom salts bought at the local health food store - plus a dash of Dead Sea Salts from Masada I got online.
I once apon a time found a site selling 50-lb bags of variously graded salt (coarse, fine, medium) but can't find it now - and the prices were good, too
I find that people who received my salt baths as gifts this Christmas were impressed with having "Dead Sea Salt" as an ingredient in their bath - they really thought they were getting something exotic and pretty neato. I didn't add much either, just enough to get it on the label!