Green peppercorns are simply under-ripe peppercorn berries. The green peppercorn is available dried, freeze dried as well as preserved in brine. The peppercorns have a mild peppery flavor and are used for pepper steak, or sauces and with vegetables.
You can typically find green peppercorns in the spice isle of your local grocery store or at Whole Foods markets. You can also purchase Green Peppercorns