You'll have to find a dealer. But they are still being made: http://catalog.buckalgonquin.com/viewitems/spade-rudders-2/bronze-spade-rudders I replace the starboard one years ago but don't remember the size. I guess your current shaft size will narrow the choices down.
Mine has one of each on it. When I bought it the surveyor said it was done as a trolling thing for fishing. I believed him then. Now I sorts think some one prolly hit something.