People who have been slaves. Slaves won't trust a rebellion from a non-slave, and they will know.

[ life as a slave is much, much different from life free. it makes a different person, most that look at rebellion and think 'death wish' besides. a spy would have to be incredibly good to fool the slaves, and then keep the story going for, essentially, their entire life if they meant to lead that rebellion themselves. the moment they're revealed as an impostor, all trust will be lost. ]

And no former slave will be willing to go back there.