The Foothold sidemission: Skirmishes supposed to have a chance to give a raised skeleton as loot. I’ve repeated the mission around or more than 40 times without a single raised skeleton. (Apart from the one from the reward.)
Considering I was able to get a handful of Gobbo Gits from around a dozen repeats of “Retreat”, I feel the skeletons are likely to be bugged and not drop at all. That, or the drop rate is idiotically tiny for the weakest undead unit in the game.