If you want new, then try Seaway Marine, in Seattle or Engine Blade and Prop, in Milwaukie WI. If you don't mind buying used, then try www.aomci.org and one of the members there might help. Someone here may even chime in and help you out too.
Part number 311189, available from any Evinrude dealer or the sites mentioned above. Or direct from BRP. http://shop.evinrude.com/ The BRP site only goes back to 1968, but no matter, the starters are the same.