Tombstone was a town in the Arizona Territory in the United States of America. The First Doctor visited Tombstone in search of a dentist, who happened to be the gunfighter and gambler Doc Holliday. He and his companions Steven Taylor and Dodo Chaplet soon became involved in Holliday's feud with Ike Clanton and his brothers Billy and Phineas. (TV: The Gunfighters)
Real world location
Though it's possible that a real world location doesn't exist in the same geographic space in the Doctor Who universe, such cases are few and far between. Thus, the map at right is probably a good indicator of the DWU location of Tombstone, Arizona.