There has been discussion among rock gardeners, roof gardeners, bonsai growers, and the crevice people about sourcing expanded shale and other permeable aggregates. These materials are like porous gravel, holding water and nutrients without organic material- which is very useful mixed into crevice garden soil media and in containers to grow alpine or rock plants.
Here is a list I've been compiling:
Permatill (by Stalite) North Carolina
Turface (actually calcined clay) or “Pro’sChoice” brand
Trinity Expanded Shale (Golden, Colorado)
Soil Mender brand Expanded Shale (Texas)
Cat litter/“Oil-Dri”- coming in bags at the pet or automotive store, these occasionally turn out to be useful expanded shale, calcined clay, or diatomaceous earth, but most of the time are just clay clods- adding water will quickly reveal the truth. A source for desperate times.
Broken pots- Terra-cotta, porous pots have the absorbent qualities of all the above, but this is not true for vitrified stoneware pots which are fired hotted and not porous. Broken terra-cotta may have been used for a very long time historically by rock gardeners in-the-know! Not a bad plan-B if nothing else is available or you want to dispose of old pots.