The baseball bat can be found in the discount store beside Ammunation on route 68, it will be lying on the floor in the storeroom.

One can also be found inside the Altrusit Cult camp during Trevors optional side mission. Right after a cut scene, Trevor ducks behind a rock, and the bat should be on top of the rock.

Another bat can be found in Northern Los Santos in a park behind some bleachers. The park has a BMX bike area.

A bat can be picked up in Rockford Hills (up and to the right of the Golf course) on the 2nd floor of a house that is in-construction.