When I am ordering metal for any reason, I usually head over to the local sheet-metal fabrication shop or local machine shop. You can usually get them to place the order for you, and since they already deal in shipping in bulk you can usually get the material cheaper. One added bonus is that if you talk machining and boating with these guys, you might even be able to befriend a machinist who might be able to do some custom work for ya in exchange for a sail on the completed project!