Radishes were found on the dining tables of Egyptians dating back to 2780 B.C. China later developed many of the varieties of radishes that we eat today including the small round red and white radishes seen in most every grocery store across America to giant daikon popular in Japan and other parts of Asia. Another similar radish is the Korean Radish. The flavor varies a bit from variety to variety but most are crisp with a slightly peppery bite. Radishes are mostly eaten raw in salads and other cold dishes but the larger daikon is also used in stir-fries.
Clearly the small red and white radishes are available in the U.S. in most any grocery store. You can find more exotic varieties at your local farmer's market. The Asian varieties like Daikon and the Korean Radish can be found in Asian grocery stores and sometimes your farmer's market too.