Try an RV supply
RV and boat fresh water systems are pretty much identical.Or better yet, find a real old fashioned hardware store. Not one of the retail "boxes" or any chain (though you might find the right size compression fittings--and btw, marine/RV fresh water plumbing is the same size as household water plumbing) at one of 'em), but kind of independent store that has all the widgets the help at the "boxes" never heard of...and where you can buy just the one you need, not a pack of 12. And the people who work there actually know what you do need, and which "workarounds" will work if they don't have the exact weird washer you came in for.Once you've found one of these wonderful hardware emporiums, you'll wonder how you ever got anything done without it.