I've been down this road once before and found that although there's no end of separate marker light assemblies (from the likes of Tomar and Precision Scale), none of them are for observation cars. However, in terms of what you can find in Lionel parts, many of their boat tail observation cars had rear roof pieces with marker lights moulded in, such as this one:
This part is still available at Lionel. At a stretch, I suppose you could cut out the markers but a better course might be to scratchbuild something similar using either Plastruct red acrylic rod or model car components. The latter are not hard to find on eBay, like this one:
It's not the bezels themselves that are useful but the red plastic sprue that can be sanded to shape, polished and clear coated so that it resembles the part you are seeking to duplicate. Acrylic rod is slightly harder to work with but holds a sanded shape better.
K-Line generally used a part like yours such as on this SP observation car but I have never seen it available as a separate piece, probably because it was part of the body shell:
Maybe more scale accurate, and simpler to make from scratch, is this 3rd Rail/GGD observation marker on one of their SP Daylight cars:
This is simply a ring mounted on a base that is roughly oval in shape. On my car there's a small red LED mounted inside the ring. The ring is actually open at both ends so the light is seen from back and front. This piece could be fabricated out of plastic tube and sheet.
