Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Start with masonry stores, look up brick, rock, soil, compost, landscape, and similar terms.
I have even heard of it being available at places like Home Depot or Lowes, so check all the hardware stores.
Things are sand blasted before painting. If all those options do no lead anywhere, try calling industrial painting contractors. House painters probably do not use it.