I think if you research the archives (older post) you will find some information about this topic..I agree, I was on the Zuiderdam last October and it was the best bed I ever slept on..If my failing memory serves me correctly it a Sealy that is especially made for HAL but again check..good luck, let us know when you find out and ask if they give a group discount
Go to hollandamerica.com
The mattresses are for sale on their site.
I haven't heard recently from anyone who purchased one but I know of a few people who purchased them several years ago and they were very pleased with the process. It all worked out very well for them.