Welcome to the forum.
I agree with what's been said. A&E parts for an awning that old where the awning has been discontinued for some time are usually not readily available. Somebody else was looking for a part for an older awning here on the forum (I think it was this forum, I'll do a search when I get a chance) where they reported back that Dometic doesn't hold any parts stock of awnings that have been discontinued which, if true, is really a shame.
It may come down to having to have a metal/machine shop somewhere fabricate something that can be used as a substitute. Or, look for used discarded parts from one of the RV salvage sites.
Also, keep searching different RV parts sites & ebay as you might come across by accident a vendor who may have one available. Here's one that by doing a quick search appears that they could have one ...I actually put it in a cart and it accepted it but you might call the number of the vendor listed on the page to confirm before going through the ordering process:
Dometic 8551202.400U Hardware, Weatherpro Std Blk - RV Parts Express
^ those are the arms that are sold in pairs (2 arms), I haven't yet seen one arm sold by itself for some reason.