Sean were are you located??
There is Starboard and Seaboard.
Starboard is a Marine board made for boats and the like, and you pay a perdy penny for it!
Seaboard is the same stuff! But made for resturants, bathrooms, Marine use, ect ect...
Comes in more colors and different thickness, was much cheaper when I got mine.
As for were to get it, depends were you live? You will have to research your area.
But if you are with in the Boston, Ma area here is were I bought mine, they may ship also? and the prices here on there site have not been updated, you will hve to call to ask all your questions.
They now have a textured finish that they did not have when I got mine, it would have been great for the tank covers you walk on! But they are not slippery at all smooth.
http://www.jfreeman.com/cpg_seaboard.html