Dragon Balls are the most useful and iconic item in the game. There are seven of them in total. Once you have found all seven, Shenron can be summoned at the Dragon Ball Pedestal. He will grant you a wish. You will get the "I Summon You Forth: Shenron!" achievement for making a wish. You need to do this several times to unlock all characters in the game. Always wish for a new playable character to unlock them all and get the "I've Come For You!" achievement. Use the following method to get Dragon Balls easily:

1. Play Parallel Quest #04: "Prepare For The Attack Of Saiyans!"

2. Defeat the first three enemies as quickly as possible.

3. A portal will appear, and shortly afterwards a group of Time Patrollers will spawn. Stay in the starting area; do not go through portal yet.

4. Challenge the Time Patrollers to a fight and defeat them. Time Patrollers always drop one of three item types: materials, equipment, and key items. Look for key items, as they are always Dragon Balls. It will display the type of drop at the bottom of the screen. If you get a key item, finish the quest. If you do not get a key item, immediately restart the quest and try again.

This method allows you to get one Dragon Ball approximately every three playthroughs -- roughly one every five minutes. You should be able to farm all seven Dragon Balls within 30-60 minutes. It works online and offline.