Stay away from food items. You will get a bunch of replies telling you to use stuff like flour, baking powder/soda, sweatener, etc. These items will look good in the short term, but over time can yellow and breakdown, or be eaten by bugs.
Best product I have found is Woodlands Scenics Snow. It is made from what is called Micro Balloons. They are tiny balls of styrene plastic. Simply apply pump hairspray to the area you want it to be, then sprinkle it on. For thicker areas, continue applications till you have what you want. A big jar of it only goes for about $8 and will provide many a blizzard. Here is how it comes out.
Spend the few extra bucks and get the better quality product and it will last much longer.
A bottle is enough for a few blizzards as well.
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