You can order a board from West Marine. Technically this would have to be permanently attached to the structure.
"The official number assigned to documented vessels, preceded of the abbreviation "NO." must be marked in block-type Arabic numerals at least three inches high on some clearly visible interior structural part of the hull. The number must be permanently affixed so that alteration, removal, or replacement would be obvious and cause some scarring or damage to the surrounding hull area."
A less expensive way to do it is to pick a spot on the structure such as inside a locker and use the 3" stick on numbers covered with a layer of transparent epoxy.