Although mossy pots are available in fashionable flower shops, you can make your own.
Source: Martha Stewart Living, Spring 1991
Unglazed terra cotta pots
Paint unglazed terra-cotta pots with yogurt, buttermilk, plant-food solution, or beer, then rub with earth; keep pots moist by planting something in them and watering. In three months they'll look centuries old.