Art supply store should have the soapstone. A flea market might also
(bed-warming stones, used griddles), as might a kitchen supply store
(griddles, but not cheap). Ideally, find a broken soapstone stove in a
junkyard.

Toss your wood mold in an oven for a while, or just do a few pours in it
and remelt them to get the surface moisture out.

There may be a problem with your sand-casting technique or sand mix if
it's "distorting when you pour in the metal"

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Cats, Coffee, Chocolate...vices to live by