where does one buy marble dust

Try an art supply store. That's where I got mine. "Q"

well remember boys, I started this with my crappy snow job post. throw some love my way!

well ziggy keep us posted on your progrees with the diorama
good luck

there is this thing called micro balloons, very convincing.

There's a feature in HF forum on the use of microballoons for snow entitled "Making Snow for Beginners" if my memory holds.

I've used bicarbonate soda in the past and it does not change color. The best way to spread it whether its salts (farting or non-farting kind), marble dust, etc. is to use the cheap and simple tea leaves strainer. You can get either the nylon or metal one at any grocery store.

Apply in thin layers. Use an atomizer (the one used to spray water on clothes before ironing will do fine) if you're using a very diluted mix of PVA:water to secure each layer. Can hairspray can be used too. Wait a few hours for the glue to harden. Blow off unsecured "snow" and add another layer.

Snow dios are generally not as portable/moveable as regular groundwork.