Keil Line....Available direct (no website) or through Walthers dealers.
....although since the passing of John Keil a couple years ago, the future of this line may be uncertain.
But, Gunrunnerjohn's suggestion is spot-on simple enough. Frankly, having people in the seats will garner more attention than the seats themselves. A few empty seats?....a simple painted white swatch on the back to represent the headrest doily adds to the credibility. A dude with a laser cutting gizmo could crank out a bazillion armrests of wood, plastic, or thick cardboard to cap the ends of John's seats. IOW, spend your time/$$ on ridership....not finessing empty seats!!! ...just MHO, of course!
FWIW, always....
KD