A Korean radish is a large (about 6" long) green-tipped Korean radish used in kimchee, salads and other dishes. The radish has a firm crunchy texture. This radish is also found in other Asian countries such as China and Thailand.
Look for it in Korean markets or other Asian grocery stores. If you can't locate them you buy seeds online at Kitazawa Seeds and grow your own. Depending on your climate they are not terribly difficult to grown.