Thread with valuable information here.

By "source" do you mean you are seeking the recipe or the ingredients for the recipe? The thread above has the recipe and also some good tips on locating the individual materials.

im sure al can tell you how to make al's gritty mix ... its a diy thing

In addition to what Popmama offered and at least equally valuable because it explains the concept that drives water retention in container media, THIS LINK provides a good deal of useful information. Keep in mind, please, that the recipe is no where near as valuable as an understanding of the concept that provides the foundation for the Gritty Mix.

Al