Soapstone, talc, and asbestos (all related) were mined around where I grew up in Vermont for years. There is a line of that type of material that runs from the Canadian border south towards Springfield Vermont. The asbestos mines are closed and the high quality talc is fairly played out, but there is still some soapstone being mined.
Vermont Soapstone Frequently Asked Questions
The good material would
not be available, but you
might be able to get some of the broken scrap, commonly known as grout if you can figure out a way to gather and ship it at a reasonable price. My best guess would be to get a shipping container loaded to whatever the safe weight limit is, and then get someone to fund the truck/rail/ship shipment.
From a logistics POV, China might be an easier source.
My guess is that the soapstone is being used as a heat storage media in the solar still project. Molten nitrate salts might be an alternate option.