Sorry don't know exactly where but if you don't get the exact answer you might try one of the
marine engine suppliers that deals with
Yanmar engines.
On the other hand, you do know that alternators are alternators? Yanmar does not have a special model made only by them that you must buy to
work on a Yanmar
engine. Depending on the output you want/need you can buy any Hitachi, Leece Neville or other well made
alternator and use that. Any good auto
alternator shop should be able to fix you up with a new, used or rebuilt unit.
Important Note: This does assume you don't have a gasoline engine requiring a spark shielded unit.