Look up plastics wholesalers or distributors in your yellow pages. I have found full sheets for the same price the boat stores sell 1/4 panels. Some machine shops too are sources for smaller "scrap" pieces.
Don't limit yourself to just King Starboard, as CPG Seaboard is pretty much identical and I get it in the northeast for 1/2 the price of King brand starboard. Both are a 'softer' (as in machinable) high-density polyethyene (HDPE), also called high molecular weight PE, but of a premium grade with UV protection in it ... and not the cheap white version of HDPE that some cutting boards are made from. Though I do find that cheap stuff a good source for some projects, e.g., spacers or backing plates that don't show.
CPG also makes their's in a seafoam color that nearly matches the Parker gelcoat.