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
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.