Where can I find plaster casting supplies for set making?

I am trying to put together a list brick and mortar stores that sell stage design materials (not lighting). Does anyone have any good suggestions? Looking mainly in the U.S, but wouldn't mind looking elsewhere.

 

My go-to for casting supplies is https://www.smooth-on.com. They sell online and through brick and mortar distributers. If you're looking for actual plaster of paris, you can get it at hardware and home improvement stores.