well, i dont know where the snow textures are but you can open ice station in UED and browse the textures...
as for making the snow, this is a technique i have used to create rain and might help you.
pic your snow texture, create a special brush which is 2sided, masked, and transparent and introduce it to your level. (transparent is a personal pref)
once you lay out all your special brushes all over the level, you want to make a zone info and go into the ZoneInfo's properties. Under "ZoneInfo" you will see lines called "uPanSpeed" and "vPanSpeed".
tinker with those until you get the movement and speed you like.
oh yea, sheet brushes for things like rain and snow can look funky if you have a sky which contrasts with the color of the rain or snow. so, what you'll want to do is get your sky close to white so the brushes will blend together when you look up at the sky-thus resulting in a pretty damn nice looking flurry of snow. hope that helps