We used to use these sometimes in refrigeration. They fit either SAE flare or inverted flare
If you can find any copper washers at all (they are getting harder) and anneal them so they are soft, you could use them as well. "Might take a couple of tries"
Parker calls them a "copper flare gasket"
https://www.amazon.com/dp/B077NHZ98C/?tag=fabo03-20
"Flaretite" is a brand name
Flaretite Seal
COPPER FLARE GASKET