I got one from Coastal Canvas several years ago. They're the OEM supplier in Beaufort, NC. Your dealer can also get one.
Don't remenber what it cost because I had a lot of other work done at the same time, but I'm thinking about $150ish.
If you get one, ask for one with the strips of starboard on the underside to hold it in place as it will fall in without them. It is a nice option to have.