It is used by car upholstery shops to go into areas like the kick plates at the inside of the cowl. I think you'll find that any upholstery shop that's doing old cars will have some or can certainly get it for you.
That said, the original was one single piece, which made it nearly 8 feet long. None of the upholstery shops I found had a piece that big. I ended up using textured vinyl, which they use for the same purpose on newer cars and which comes in 4' x 8' sheets. It looks great, has the same pattern as the cardboard did, was easier to bend for the corners (just use a heat gun) and will I'm sure wear better than the cardboard. Slightly non-original, but it's not visible when the trunk is closed.
Hope that helps.
All the Best, Chip
