Busch and Noch both make snow powders in their model railroad lines but I've never actually seen them. My friend just picked some up to try but he neglected to tell me the brand! He did say that there are tiny reflective crystals in it that give that typical sparkle you see in a snowfall. There is also a Japanese product but since my Japanese, other than a dozen words or so, is almost nonexistant, I have no idea what it is! It's demonstrated in this video.
(Check out the other videos in the series while you're at it. They're quite interesting.)