My daytime job is Controls Engineering Manager for three sawmills in Idaho. As I have worked in them, one thing I have noticed is the different textures of sawdust. The sawdust tends to get sorted with the sawdust carried in the air causing a normal sorting by size, the coarser sawdust falling nearest to the saw, with the fines drifting farther away.
In a recent visit to one of the mills, I found a large collection of really fine sawdust, almost the consistency of flour. This seems to be more in the line of scale sawdust for dusting on top of equipment and on the floor. I collected over a gallon of this and realistically could gather sawdust of coarser grades if it would be of interest.
If you would be interested in having some fine sawdust for your modeling, please let me know and we can likely make arrangements. I'm not out to make a lot, but certainly want to cover my costs.