I remember seeing in one of the books in my library a how to create your own using a fine grain film and shooting various textures.
I can't actually remember perfectly but the process involved either underexposing the film or under developing it.
To simulate some film grain, you might get away using a picture of some concrete or something like that.
Why not just use the alien skin exposure plugin? This thing does exactly that, simulate various film stocks.
Or you could do it with an action script by adding noise to a layer and setting the transparency level fairly low.