Unless you've checked with a dealership parts department for that specific part number and found nobody in the country has one in their inventory, I wouldn't give up the dealership system yet
. Even though the part has been out of production for a long time doesn't mean that an old dealership doesn't still have one of the shelf somewhere.
I have found obscure door window parts for my 73-77 a-bodys by calling a local GM dealership parts dept and he found 7 or 8 dealerships in the country with the part, even though I couldn't find it anywhere online. He gave me the names and phone numbers of the dealers and I stocked up on that part.