Get missing spare parts where??

Where do you get missing spare parts (NEW) that you don’t get from the scrap dealer if you want to build new parts. Does anyone know a few dealers?? At Audi-Classicpart there is also no part. 8A0 853 968A 3FZ – bar for door rear RE —————– Man can’t even build an accident with his car because there are no parts anymore. 🙁