It is a Sealy made for Holland America, but you can buy them from HAL through their on-line store
A couple of people on another board have bought the mattress set and said they were wonderful - not exactly like the ones on the ship, but close. They have HOLLAND AMERICA printed all over them.
I was going to get one, but being a Canadian would have had to pay duty taxes and it turned out to be far too expensive.
We had them on the Westerdam and they were wonderful - best night's sleep I ever had.