There are many Slot Releasers in the game that can obtained through a variety of means. Slot Releasers can be bought from Moogles, obtained for completing a mission, finding one in a chest, or redeeming them for Mission Crowns and Challenge Sigils. They are most commonly found by completing missions.
There are a grand total of 105 Slot Releasers that can be obtained in the game. Each time Roxas is assigned to a new rank, a Slot Releaser is available at the Moogle Shop.