I would hazard a guess that there is more than one gasket in the engine.
Knowing which would help in pointing you to a source, but there are places that carry a wide range of parts and may be able to help you. For a start:
Shipyard Island Marina
If you have the Yammie outboard with the jet conversion, they both should be able to help.
You may be able to find the parts on one of the exploded parts lists available on the Yamaha website: