It's very unlikely you will find a cover for your specific application. Will the missing breakers be at the bottom of the panel? (I'm assuming so) I've seen the local Rube Goldbergs make metal plates to cover gaps in the panels and secure them with self tappers. I don't know if it's "legal" and it may just affect the UL listing of the panel. But I have seen it done.
Guess that fall into the "better than nothin'" category.
Do you want it your way or the right way?