Yes I beleive it does come as thin as a quarter inch in thickness. And you can add to any thickness by gluing multiple layers. It is the tightest cellular rubber on the market that I am aware of. And it can be painted, or sealed etc, but you wouldn't need to in the floors, door panels, trunk etc.
However most of the commercial shops will probably only have half inch or above. But you never know, its worth a shot.
1989 420SEL 105575 miles
1990 Chevrolet E-Cab C-1500
1989 Olds Toranado Trofeo'