From the Super Mario Wiki
Mushroom Balls are items found in Mario & Luigi: Bowser's Inside Story during the sidequest Mushroom Ball Derby; they are balls in the form of red mushrooms. There are 16 total, each holding their own special prize if Mario and Luigi hands them to the PR Toad of Toad Town Mall. They are found around at the mall in hidden blocks and by spin drilling underground.
There are 16 rewards, one in each Mushroom Ball. The rewards found in the Mushroom Balls are as follows (they will always be in this order, independent of the order of the Mushroom Balls):
When near a Mushroom Ball, a question mark will appear, indicating the location.
Mushroom Balls in invisible blocks:
Mushroom Balls buried in the ground:
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