Make your own using nitrile rubber. How big/thick is the washer? I probably have some in my project junk!
Check a hydraulic store, I bought thin aluminum washers that had a rubber impregnated ring on them to fit my fuel cell.
I'm pretty certain it's just an aluminium washer. Aluminium or copper should be fine. Or, as already suggested, a bonded washer / Dowty seal would work too. All readily available and plentiful online. Chris
Napa/O'Reilly/Autozone etc usually have a good selection of copper washers, just take the fitting down with you and match it up
https://www.ebay.com/itm/166658340338? select 5/8" ID, get 5 for a dollar plus two dollars postage. And wait a week. Or look in your junk bin for a 5/8" ID washer, or gasket, or just make one with a single ply cardboard box and a pair of scissors.
You might look at Auto Zone. Here they have Edlebrock inlet hose barb fittings and tube flare fittings with the washer.