Looking for brake fluid reservoir cap

[OleSenna]

I'm leaking fluid from the brake reservoir on my '72 K5. It's got a crate 350, but uses the original power brake system.

The reservoir has the rubber gasket with the double-baffle insert tucked inside the "hood" that gets clamped onto the reservoir. I'm striking out at the local (Atlanta area) NAPA and AutoZone-type stores. They carry the entire master cylinder assembly, but not the rubber gasket insert.

I realize I can try a junkyard, but I'm wonder whether any board member sells these parts or has a better idea.

Thanks in advance!