If the question is solely provenance, Zachys is probably my first
choice. But they are seldom anywhere near price leaders on older
bottles.

Sherry-Lehmann has good storage but again tends to be a bit high, but
currently has the '75 Gruaud-Larose at a good price ($90 I think). I'm
putting together a G-L vertical offline and was just checking their
Gruaud prices (I already have the '75), so unsure what else they have.

In addition to K & L, other merchants that tend to get mentioned in
discussions of older bottles include Premier Cru , John Hart, &
MacArthurs/Bassins. Sam's, Calvert-Woodley, Wine Exchange, Rochambeau,
etc. tend to mostly have more recent vintages in my (limited)
experience.
A google search on this group would show some past discussions.