I don't know if it's still made, but I got some sheets of printable fabric at WalMart years ago made by June Tailor, came in white and beige. If you can find a rug pattern you like this material would be good for rugs.
I also used 2" wide cloth athletic tape in my Weaver N&W cars. Sticks to the metal floor and then you can apply whatever color paint to it, gives it that carpet look. I haven't put any seats, beds, or walls in them yet but when I do I guess I'll need to cut the tape so they will stick where I want them.