According to CM's at the Boneyard, the little pebble stuff is called Texas Grit. They were unable to tell me where it comes from ... but my DS adores the stuff, too. I laugh b/c I swear, with every trip we've taken and all the grit that's ended up in pockets, we could have filled a sandbox if we had bothered to save it!
Good luck finding it ... maybe you can do an on-line search for the stuff!