You mean haydite? I was looking for that a while ago & the only way I could figure out to get any was by going to one of the places that makes the stuff (this was in Indiana). I was using a finer grade, which turned out to be somewhat less useful than I expected because of a large proportion of dust, though presumably this wouldn't be a problem with greater than 1/4" material. An easier-to-obtain material similar to haydite is turface, which also has very low dust content even at the smaller grades. Basically, haydite is expanded fired shale, turface is expanded fired clay; the only observable difference is that haydite is darkish grey whereas turface is light brown. I've ordered turface online through hummert.com ; you might also be able to get it locally at places that cater to lawn maintenance & landscaping people. It's most often used in baseball fields & the like in order to provide a well-draining & non-compressing base for turf.

Patrick Alexander