Hard to say what's local without knowing where you are, but Total Plastics Inc on Pulaski Hwy 866-658-8167 in east Balto likely has what you need. I can't say for sure if they carry Starboard brand, but last time I checked they carried the equivalent material. They'll cut it to the exact size you want as well.
2nd - Maritime Plastics 910 Bestgate Rd Annapolis 410-263-4424 , I purchased a few pieces I needed from them.
Really nice, cut it down for me as I waited. Was just some simple rectangle pieces that I needed, but was a pleasant exchange.