From what I've seen, most of the chicken grit is in the 5 to 7 lb range, I have seen some 25lb but it is about a buck a lb., with the exception of Gran-I-grit, about 25c a lb.
I don't know what your specific application is, but Al's gritty doesn't have to be granite.
It is about it being inorganic, non-porous, and the right size.
The next large quantity that I know of is oyster shells that are also used for chicken grit. The sizing is a fair bit more inconsistent. Oyster shell have calcium and ? in them. I don't really know about whether or not they break down over time and if any calcium released in usable, or over-usable form by the plant. The gritty mix recipe does call for adding gypsum.
I've tried calling local rock and stone sales that is used in construction and landscaping, no luck. There just doesn't seem to be any demand for 1/4inch and under graded rock. Give it a go, maybe you will have better luck in your area.
I'd love to find another option, my supplier is a 1.5 hour roundtrip. I bought a dozen bags last time to save on trips, so I'm good, for now.
Good Luck.