I would say eBay as well, just text everyone that has whole car breaking or the part that you need. As Blue_Thunder said, make an offer all the time, as eBay charges are huge, so everyone add them on a listing, but if you are happy to pay in cash and collect the items yourself you can make a deal.
And if you are looking for some expensive stuff like Recaro's, or S4 bumpers etc. you need a luck to find someone who does not know what he's selling and gives for A4 prices. It does happen sometimes. Not for me, but it does.
And that wouldn't be eBay I guess, scrap yards, private breakers mostly.