Chemically Plaster of Paris is Gypsum that's been heated to drive out some of the water . But you just cant go down to home depot and buy some and throw it in your wort because its prob been treated with all sorts of stuff and prob has lime in it. Its got to be food grade. so its going to be cheaper and easier to just buy it online for a few $.
I guess I'll just order some. Being that I live in the middle of nowhere I'm always looking for alternatives. I have to think at least a week in advance if I need something, so I have had to get good at getting by.