If the main issue is a home inspector gigging you for that, I'd leave it just the way it is. There are quite a few boxes that come with no hinged covers and exposed breaker toggles.

It doesn't look to me like there's exposed wiring or similar hazzardous condition there, so I can't see why a home inspector would be justified in calling it a problem that must be fixed.

But if you need to fix it, it might be easier to cannibalize a used box than track down the part. You can very likely find a similar used box on Ebay for pretty cheap. If so, throw away the guts, treat the cover to a can of spray paint (if necessary) and you're good to go.

I can't see why he should have a complaint.
There are no open slots, so I would say that it is Code compliant.