Crushed granite is frequently available at roofing supply companies. Usually they cary various grits by A-1 Grit Co. granite. Go to
A1 Grit to see their products. The #10 grit is a great top-dressing and the #16 and #10 both make nice additions to a mix.
PS the chicken grit mixes with oyster shell are actually nice additions to a mix as they help add calcium. My main objection to the chicken grit is it is more expensive. Lots of luck. RON