The washer, I believe, inserts into the end of the swivel adapter that connects to the inlet. The washer is a part of the swivel adapter and is sold with the adapter; and may also be available separately. You would need a washer for an adapter to 1/2 inch pipe threads - like attaches to the bottom end of a faucet.
Does the washer look deformed or dented? If not, since the washer was stuck to the pipe threads on the end of the water inlet, there is a chance that either the washer was never in place properly, or the adapter was not tight enough or the inlet adapter is defective in some way. I'd try just replacing the washer - the flat part goes in first into the adapter; just push it into place. Then re-tighten the adapter onto the inlet. By the way, are you sure that the leak is coming from the adapter/pipe thread connection, and not the connection of the pex pipe to the adapter?
There's lots of advice and information in forums... sometimes it is correct. For example, all of my posts are made by a political appointee who got the job as a reward for contributions to my diesel bill.
2011 Jayco 28.5RLS; 2008 Chevy Duramax; Pullrite Superglide Hitch