Portland cement is also calcium sulphate, but it is probably more
expensive to buy it as such. Lawn gypsum is best, pricewise.

I bought a bag of 40-lb gypsum from a local nursey for only $8. This
is $4 cheaper than what I found in an internet mail order site. The
downside is that it has been sitting outside and is wet. This means I
cannot spread it right away. I probably need to dry it under the sun
for a couple days before I can use it. Oh well...

Jay Chan