Hi Ned,
FWIW: I stumbled on a reference to this link and thought you might (or might not) be interested:
http://www.purearagonite.com/co-opprogram.html
They say they sell and ship "Pure Caribbean Aragonite" in 40 lb bags at what seems to be cheap. If I did the math right (including shipping):
35 bags @ 40 lbs, approx: $15/bag, $.38/lb
70 bags @ 40 lbs, approx: $13/bag, $.32/lb
Of course no one person needs that much sand, but a store might!