Most of the diorama details come from exchanges I have done with other folks who build dioramas, but I also have an extensive diorama parts box where I keep all the stuff that I think will work . . . but believe it or not you can get a lot of mileage out of simple paper.
Like the discarded Mexican blanket inside the Challenger that was found on Google Image, printed down to scale on regular paper, and then glued to the interior . . . let me dig up the picture . . .
To make the interior look thrashed, I just glued pieces of Kleenex to the seat with watered down white glue and then after it dried rockhard, I cut it and pulled it into the right sized tears . . .
I had done the same thing to the headliner . . .