"Chicken grit" is actually crushed granite and really is for chickens. I get mine at our local feed n seed store. It comes in different sizes from the size of sand to a medium size.
Nancy B

Lee,

As Nancy illuminated us with, poultry grit comes in different sizes, from #1 (the smallest) through #6 or so. I use the #1 or #2 for a soil mix. You should be able to get it at a feed or agricultural supply.

Below is a picture of the one I use...like Jeff and Nancy B mentioned the best place to look for it is at your local Feed Stores...

Here's what it looks like...I usually rinse it before using.

Here is a link that might be useful: Manna Pro Chicken Grit