Save a search for "Atlas Berwick" on eBay. They pop up from time to time. I managed to find two ATSF and one rare-as-hens-teeth TTX this way.
If I sold models as sought after as Atlas O's Berwicks, Trinity hoppers, and intermodals, just to name a few, I'd be producing runs of them a bit more often. I know rarity keeps values high, but these pieces selling for 2-3 times msrp should be indication that the market exists for them, right?
Edit: should mention I'm not looking to sell at the moment (too few are and for good reason, these are fantastic pieces). I just wanted to share the strategy that worked for me.