I make a similar mix with turface, pine bark and Grani-grit. Any
feedstore with battered pick-up trucks parked in front of it will
carry grani-grit, at least in spring (it is a granite gravel that
comes in several coarseness levels to be used in different ages of
chickens). I get the coarsest type. My only criticism of it is that
the color (a bluish white) clashes with the turface a little bit.

It's always entertaining for me to buy grani-grit, because the owner
of the store always wants me to discuss my chickens.

Nina