Substitute for Chick Grit?

[_Wildwood_]

Can anyone suggest a good substitute for chick grit? I'm currently buying it from TSC for $5 a bag and they go through it pretty quickly (I have around 50 chicks), so I'm looking for a more cost effective alternative.

Thanks!

 

[azelgin]

A nearby creek, or, riverbed with sand and gravel? Masonry supply yard? Landscape supply?