Most Attack Pieces can be found in and collected by striking blocks that resemble them, though others can be obtained by different methods (usually from NPCs after completing certain objectives). At times, Mario and Luigi can get either one or multiple Attack Pieces at once.
Mario obtaining an Attack Piece in Mario & Luigi: Dream Team.
Attack Pieces make a return in Mario & Luigi: Dream Team, but this time, they grant different types of special attacks. In the Real World, Attack Pieces grant a Bros. Attack to one of the brothers once all ten are collected. In the Dream World, they instead grant a Luiginary Attack to Mario. The brothers obtain their very first Attack Piece in the Real Word in Mushrise Park, given by a Toad after saving him from Thorbs, and in the Dream World in Dreamy Mushrise Park.