honestly ebay has been so good to me. just look up the part number you need and go to ebay.
i also find a lot of my parts at the local napa. impellers, gaskets, thermostat,........... all mine are omc units though. not sure about yamas, but check.
Sounds like HE needed some BASIC TRAINING.Gaskets for the spark plugs? Haven't seen a gasket on a spark plug in 30 years. Oil drain plug and gear lube plugs are low pressure. You could use a decent o-ring form the auto parts store.Rg
All spark plugs come with a crush washer- is that what you mean? Theoretically you don't want to reuse a spark plug that's been torqued once but everyone does.
ALL?Not mine. Anyway, it seems a CYA from the manufacturer.Rg