Rebel captives are held in forts, the bigger the better. Akkra garrison in Alexandria holds up to 8 prisoners, so you might want to look there, but make sure you initiate the DLC's main quest first.
The game will stop spawning rebel captives once you reach a certain point in the DLC's main quest. If that happens before you have unlocked the trophy , you need to disable level scaling, then fast travel between forts that usually hold captives. They will start to spawn after a couple tries. Make sure you free rebels, not civilians!
Check the trueachievements guide for video walkthroughs if you need further guidance.