There are three sets of Dragonballs in the universe. A set of Dragonballs on Earth, a set of Dragonballs on Namek, and the Black Star Dragonballs that are scattered throughout the universe. You can go searching for the dragonballs without a radar or scouter, but it would take a very very very long time. Obviously searching for the Earth or Namek balls would take significantly less time than looking for the Black Star balls but that's because they of course are all over the universe. A scouter helps to find the balls a little faster but to find them the fastest you need the corresponding dragon radar. Refer to the Items page for specifics. As with everything there are limits to what the dragons can grant and they will reject any wish that is out of their range of power. Here is the information on the three sets of dragonballs: