Used Roofing Slate

Looking for a supply of used (or new) slate roofing shingles. My wife is an artist and (http://maplered.com) uses them to paint on. Approx size of 12" by 16" or thereabouts. Flexible on size.

We are located in Dundee, NY near to Rochester, Syracuse, or Elmira.

Can anyone help or point us in a direction to find a supply?

Thanks