There are several applications I'm looking at, but all involve
galvanically isolatinng the head of the fastener from the item being
fastened.
Two examples that come to mind are attaching aluminum hatches and
stanchions to the deck. In this application, only the head of the
fastener comes into contact with the aluminum, but there is enough
force that a viscous isolator, such as silicone or tef-gel, probably
won't work in the long term.
As for sizes, my current applications involve #14 oval head screws or
1/4" machine screws.
As for McMaster-Carr, I had spent a good amount of time looking on
their site before posting. Many types of washers, but I can't identify
the ones I'm looking for. There are cup washers for pan heads, and
finish washers for using oval heads on a flat surface, but nothing like
the washers I'm looking for.