I like double strength glass for a sound board, a little more raspy and loud compared to the single strength. I have also heard some really good calls with wood. Woodcrafters in Roanoke has a really nice selection of thin wood that would be great for a soundboard.
When clamping the slate and soundboards in the calls I have made I have used spring clamps. I tape a piece of cardboard to each side of the clamp and use three clamps on the soundboard and then the slate to even the pressure out.
EDITED
Just remembered you are no where near Roanoke
If they don't have a store locally they are on the web, also
http://www.rockler.com and
http://www.customsawing.com have thin board of differn't species.