Anyone, Circus cars.
Lionel or Menard's. Rubber roadbed for your "O" Gauge tubular track.
Look into Midwest Cork -a rubber/cork composite, its tapered at the ends, looks nice and its easy to work with. Scenic Express sells an all rubber roadbed. I don't see another source of roadbed coming.
What I'd like to see in 2018 is some scale size realistic looking operating accessories that have a smaller real estate profile and some of the fallen flag Eastern "coal-road" box cars that are difficult to find. .