Obtained by opening rewards mailed to you after you release prisoners (there is a mailbox on the island). You can release 5 prisoners per day for around 300-900 pirate coins per prisoner. The main room has around 3 prisoners at any given time, however you will have to contend with other players. There is a secret area just outside of this main prison-room with two extra prisoners, so you might try your luck there. To get here, just walk through the metal gate and down the stairs.