I have one of the easiest ways to find dragonballs. In the Z-battle gate, in the Majin Buu Saga, go to the fight where you are Super Saiyan 3 Gotenks fighting Super Buu. When you start the battle, knock Buu off with the up + X combo. Then go directly backwards so that you are on one side of the building in the middle of the lookout and Buu is on the other. Fire off a ki attack so that it destroys the building. (This attack will go in the direction of Buu, where ever he is, so just make sure that the building is around the middle of you and Buu.) On the center pedastal of the building you just destroyed will be a dragonball of whatever number of stars. This will always work.
Hope this helped. Cao Ren out.