There's an assignment in the mountains called Truly Divine that lets you catch true angel slaves. The race, again due to lore, can't be slavers. To get a true angel it's not enough to crit the assignment, all your 4-slaver team needs to have a special set of abilities, and be in the right order: 1) a former paladin/purifier of the light you have to get from the Plains and train into a slaver; 2) someone with corruption magic; 3) someone with biomancy; 4) someone with the Trainer trait.
I may be switching 2 and 3 around in order, but the assignment will display a bonus if you're doing it right.