Yeah, Home Depot's or Lowe's should have something like that. You can also use plain old bathroom slate tiles that haven't been painted or anything. And some fish stores carry bulk rock that is sold by the pound, so maybe you can find some good pieces there.
How old/large is the blizzard in question? When they're young, they often have that grey-ish coloration that brightens with age.
How old/large is the blizzard in question? When they're young, they often have that grey-ish coloration that brightens with age.