I was gonna suggest tap and thread if you didn't already... and it's double tapped haha
I don't think you can find a copper head gasket, you'd have to make one. It could be out of fine gauge though. It just sounds crazy that you have a leak anywhere still based on what you've done.
Maybe something to try is get some prussian blue dye from the craft store and stick the head on there with no gasket - it'd help you see if there's a gap/crack/divot some where you didn't see. Seems kinda odd that it's don them on two.
Try different head orientations as well... idk... just thinkin' out loud.
One other thing you might try is a stock puch hi torque head - the ones that are supposed to seal without a gasket. If you've got a clean one of those, the fine aluminum rings in the sealing surface should at least get it sealed up.
Are you using new spark plugs? I heard one time crush gaskets on plugs are a one time deal (though, I don't really believe that). Still something else to try though.
I'm just brain storming out loud... no pro advice or anything. Just what I'd try (edited)