Mine did not have either the sink divider or the cutting board that fits in bait prep area. I don't miss them as I only use the sink for washing/rinsing.
You could probably order from the dealer but I'm sure you would get sticker shock! I think it would be easy to make what you want and cheaper by far. Make a template out of cardboard. Choose your material. Cut and sand. As long as the edges are smooth, It'll be fine. Starboard would be what was original but you could by a plastic cutting board from Bed and Bath and use a jig saw to cut it to fit. Wood would also be an option and maybe easier to work.