I just ordered a big quart+ bottle from Hach Chemical company for $30.  Not
the cheapest, but certainly cheaper than Dupla.   I have found I could 
get Magnesium Carbonate from local supplies, but they only had marble 
chips for CaCO3.  Interesting, Hach only has CaCO3, because it's used as 
test standards, apparently.  You can take the safety info they send along 
with each chemical, and post it on your refrigerator.

some info from George about Hach, which will someday extremely soon be
back on the Krib: 

For much better service, try Hach Chemicals.  They take orders over the
phone, they will take VISA/MC, they ship right away, they also have an 
excellent catalog, they have an excellent water chemistry analysis 
handbook (free) and I guess they don't have as many lawyers working 
for them. 

Hach Company
PO Box 608
Loveland, CO

phone (800) 227-4224
in CO (303) 669-3050
FAX   (303) 669-2932

        - Erik

Erik D. Olson                                                  eriko at wrq_com
WRQ, Inc.