[X] The Great Houses, as it might help you accomplish the task of killing two birds with one stone once you have interrogated and studied the slave soldier in question. The reason being that they are the mostly likely to have some besides the garrison itself, and competing interests could allow you to mask the disappearance as pre-existing internal divisions (Sons of the Harpy, Spicers, et al). And the Peacekeepers likely don't need enhanced soldiers to deal with brigandage, nor are the merchants liable to have what you're looking for.