To get Dragon Balls you have to play story mode( I'm not sure if vs. mode works) and some times you can break a building, the dragon balls are hidden in there, so you get the dragon balls automatically.
Best way is to play the Dragon Ball GT saga. Play against Nova Shenron as Son Goku (GT)... In the beginning of the bout, fly into as many buildings as it takes before finding a dragon ball. Most...
You can find them in the recommended section of Mr. Popo's skill shop or if you are lucky, the gamble section. If that doesn't work, sometimes when you play story mode you get lucky and find a...