Unfortunately, these armrest backing plates are not available except through swap meets, salvage yards, ebay, etc. I see them fairly regularly on ebay... the only problem is that most of them are toast to begin with. I got lucky at Chevellebration a few weeks ago and bought a straight set, uncracked and unbroken, beautifully rechromed, with mounting clips, for $50. American Plastic Chrome had a booth there. Even though they are essentially no longer in business and certainly not taking new orders, they were selling off old stock items. It probably would have cost at least that much to have my cores rechromed somewhere else. The way it worked out, I got to see exactly what I was getting for my money, with no waiting, and I get to keep my cores for another day.
Keep checking ebay and elsewhere, these parts can be tough to find, but not impossible.