You will need to be specific in just what part of the charging system you need. Motorcycles have rotors, stators down inside the case. Then they have additional components that convert this energy into something usuable by the bike-this is the voltage regulator/rectifier. When you say "altenator" I don't know exactly which part you are talking about. And from a bike from the mid 80's it could be any or a combo of any of the 3 major components.
Now as to where, as others have said, ebay, bikebandit, and I would add online salvage yards. You could also try www.electrosport.com. They sell new electronic components for bikes at great prices. Ebay and salvage yards are gonna be used, but cheaper than new prices. But you take your chances with something that is as old as what you are already replacing. bikebandit the parts will be new, but much more expensive
