Stacker 1:
Collect all 3 "Backward Rear" Secret Codes.

Stacker 2:
Given by Mac after you have Stacker 1 (high friendship level is required)

Stacker 3:
Check a container in Pirate Valley.

Stacker 4:
Check a container in East Sea West Coast (after Anemos).

Stacker 5:
Check a container in Spirit Lake B1.