Aqua Pebbles by Gardener's Supply. $9.95. Aqua Pebbles are made from clay that's been fired at high temperatures, creating a porous material that holds water and then releases it slowly as plants need it. Commonly used in hydroponics and terrariums and as a potting medium for orchids, the clay balls are also ideal in container plantings. Place a layer of Aqua Pebbles at the bottom of a planter, then fill the rest with soil mix. The pebbles absorb and retain water...
You can buy the pebbles from Amazon.