Usually I would just do a let me google that for you. But instead.
http://www.dogfightink.com/MiscCustomDecals.html
pew.
Just try to remember that if a seal saw you wearing something like that without having earned it, it might annoy him. Not that I think he would do anything about it considering they are highly disciplined individuals. Just saying.
As for a stencil.. You print the picture. put it onto a piece of heavy card stock. Carefully cut out the edges with a exacto knife, then use spraypaint to finish it up.