Any clue where I could buy one like that?

Actually, I'd rather just widen my 5.7M to 10'. Is this possible without resorting to a dolphin striker? Having the mast oriented vertically with internal stiffeners seems to create an incredibly strong section, but could it handle two more feet? Would adding 1' extra to each side of the beam be better than cutting down two beams and joining them in the center, or is it such a common extrusion that I could salvage one from a mast?