A really easy and cheap snow substitute that I have used in several pics is a coarse open-weave white fabric I found at a textile store. It probably wouldn't work for extreme close ups, but in general pics it looks like rough, icy snow. It was only a few dollars for a couple of square metres. The only other thing I have used (for an outdoor pic) was a couple of kilograms of flour, which simply hosed away when I was finished.