We bought the one from Paws Aboard, I had to fab some different mounts for our boat because it didn't fit our ladder, worked out better anyway because then we could use the ladder still. Here's their link...
http://www.pawsaboard.com/ladder.asp
They are available at a lot of stores, Overtons, Boaters World, West Marine. About $200 but with a 130 lb. dog, it's worth every penny!