Couple of options but both will require some time on email or phone.
Do a search on hobby shops in the areas where teh N&W ran as they would have the best chance of ordering the cars in the first place. Call or email those that don't have a website. Youshould be able to find many dealerson the Atlas websites, Walthers (they usually list who they shipped the product to), and this website (Trains).
Contact larger hobby shops out of the N&W area that may have ordered some to fill a minimum order requirement, they may still have some on the shelf.
Rick
Be aware the Atlas cabeese have the N&W hamburger herald or the block NW on them, so this precludes their use to other eras, unless you remove the heralds. They are nice looking units and run fine of course. BLI has just released some N&W, and I can't remember if Roundhouse did or not right now (I know they did VGN). The old Roundhouse 4 window jobs can be made into a decent N&W rendition.
There are several on that certain auction site right now.