I've been trying to track down Hydrocal White in SE Michigan, to use to make plaster bats. I contacted the manufacturer, who sent a list of only four suppliers. After calling around, I have found that Hydrocal is available for purchase (by the individual bag, you don't need to buy a whole skid!) at:
1940 E. Forest Ave
They have a showroom, and the hours are 7-5 M-F.
It's hard to find, so I thought I'd share the info.