If you ever go to the LA area, M&K Metal in Gardena is the go to place, or at least it was when I lived in LA. Onlinemetals.com is easy to deal with and will ship, but not cheap. Or just Google “Metal Supply” and your location.
You didn't indicate what the backing plates were for, nor if they were an original part.
If an original part, you might give Bud Taplin a shout. WESTSAIL PARTS CO.
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