Why would you look for a replacement grille? An accordion repairer should (with quite a bit of work) be able to repair it completely, because this old grill is very likely made of celluloid and that is the one type of "plastic" accordion repairers can really do miracles with...
As a fairly large piece is missing it is also a large job, but as the mirror image of the missing part is still there (at the other end of the grille) a repairer can make a mold for the missing piece and then make it out of celluloid. It can even be done so you would never know a repair was needed, but again, it is a *lot* of work.