Its a baggage car not a coach. About the only place I know that sells something close is Labelle woodworking. Their HO-3 1905 baggage car is almost a dead ringer for that car.
Oops, just noticed this is in N scale so the HO models won't work. You will probably need to find a 60-70 ft heavyweight passenger car, cut away the sides and build new sides from scribed styrene.