We didn't get the cover with our 14' 28BH, either - I talked to the dealer, and was told it was an option, and they could order one, but it would be about $50...
What I did instead? Went to a local restaurant supply store, and got a large plastic cutting board, made a template of the sink with some cardboard, and cut the cutting board to shape with a jigsaw.
Few minutes with a palm sander to smooth the edges and round off the corners; and voila... Overall cost was about 30 bucks, and maybe an hour's work; and have a sink cover/cutting board that's a lot more substantial then the factory one.
2014 Grey Wolf 28BH, behind a 2015 F350 Diesel Crew Cab
Nights Camped 2016: 44
Nights Booked 2017: 11
Nights Camped 2017: 6