I've been working on upgrading the Vibranium entry here on Comic Vine and for some reason, I seem to remember reading a comic story that Captain America fighting the Black Panther--way back in World War II and when Cap was still using that triangular-shaped shield. Later, they exchanged shields afterwards. I thought it was implied that the Black Panther's shield was later used to supply the vibranium that Dr. MacLain used to create Cap's circular shield. Or am I simply not remembering it correctly? Does anyone know?

Has it ever been explained exactly how the US got the Vibranium in Cap's shield?

Probably from Wakanda.