see if you can setup an account a composites one and buy during will call hours. You can probably get 5gallon pails of vinylester from the Bristol location. I used to buy iso polyester and vinylester pails from the west sacramento location, but each location is different. Also check your PM's for a 20sf listed for sale. I bought two 3/4" 4x8 penske board sheets from composites one in bristol. If I were you I would just use acx plywood from home cheapo or splurge for marine ply from BoulterPlywood in Medford. It'll be good for another 30 years.
http://www.boulterplywood.com/MarinePlywood_4.htm