The most popular material for such uses is Savage Translum (in the US). In Europe, Translum is sold under the Tetenal brand name. (Canadians will have to import it themselves in any case; neither Savage nor Tetenal distribute it here. It is equally difficult to find in some other regions, and while the stuff is cheap enough, shipping can add up.) It's a thin, semi-rigid PVC sheet (200-300 microns) usually sold in rolls as both a background and diffuser (thus the 54" width).

Translum still requires separation from the subject in order to be lit separately, but because it is backlit and a good diffuser, it produces very clean, smooth gradients with no visible texture. It's a little trickier to handle than seamless paper as is, but you can take the curl out of it effectively with a hair dryer and a bit of patience if you don't have a weight bar for the bottom.

You can also use styrene sheeting meant for backlit signage. It generally is too stiff and fragile to roll, though, and difficult to store if it's not framed. The upside is that it's available just about everywhere once you know what you're looking for - if there is no jobber you can visit locally, there's always a signmaker handy who can get it for you.