I've been using sawdust from my cabinet shop for about two years. Most shops will save it for you IF you take them a large trash can or two, make it easy for them to dump it in, don't bother with liners. Expect some "trash" from where they sweep it off the floors. Easy enough to pick out though.
The finer sawdust like from the wide belt sander cakes up but the larger dust from table saws, routers, or planners stays nice and fluffy. I keep about 6 to 8 inches in the coop and out in the run, although the run cakes up when it gets rainy.