For what it's worth, I bought a piece of Starboard from Cabelas. Normally I would look around locally, but I had a gift certificate. It was pretty painless to have UPS drop it off at the house though. Something to think about in case you have trouble finding it. Unfortunately, I'm not sure about finding the dimensions you're looking for.
Cabela's -- King Starboard Marine Grade Polymer
Cabela's -- King Starboard Marine Grade Polymer
