Nonstandard Return Air Grille needed 12" x 5"

I can't find a 12" x 5" return air grille anywhere. The major stores all carry 12x6, but even the small local hardware store that got me all the other non-standard sizes ones can't get me this one. Is there someplace online where I can get this? A 12x6 won't work unless I fold in the uppermost tab. The existing ones were painted over at least 5 times (had to chisel to get them out) and one was bashed in. So refinishing is not an option. If it matters it's from a 1950 Baltimore rowhome.