The answer to your question is "there must be" but possibly he or she is someone with a 3D scanner who could copy your existing end piece in a CAD drawing you could have 3D printed. Or you could check on Shapeways whether someone offers an HO version of these cars that could be scaled up, which I have done for UP auxiliary water tenders that no one makes in O scale.
The trouble in both cases is (a) cost and (b) you end up with a part in featherweight plastic that is not easy to match to an existing passenger car body.
My first thought was actually Union Station passenger car models, who make the flat ends of these cars in O but not it seems the round corners and roofs: see http://unionstationproducts.com/_4096_4.html
P.S. The HO scale roof piece is here: http://unionstationproducts.com/_tsp_504.html
Lionel's forthcoming UP 21" UP excursion car set is meant to include a blunt end dome observation car. If you are committed to an 18" set you obviously won't want that however.
Good luck!
