Walnut usually has a grit size range, I believe 10/20 is common, 20/40 is finer and dustier. When my mix got dusty I winnowed it, pouring from one large container into another(2 foot dropon a windy day). It worked quite well. One of our local landscape suppliers(deco bricks,etc.) has walnut in 50 pound bags, correct grit, about $1 a pound.