Where can I find a cover for this external breaker box (1961 SFH in CA)?
Uh-oh, it's a Zinsco!
Unfortunately, just buying a new door isn't enough to restore this panel to a serviceable condition, as it's a Zinsco (from the design of the breakers), with its attendant breaker-to-busbar contact issues.  If that wasn't enough, independent testing has shown Zinsco breakers to be both miscalibrated and unreliable, with issues with jamming or simply failing to shut the power off when switched off.  So, I'd have the whole box replaced with something of a current make and model that won't fail in such a way.