Without being too spoilery here: in the swamps, a special mob will randomly drop them when they die. So you can go to that same spot and farm the mobs to get them, and there are clever ways to kill them instantly. You can figure it out.
For me it is just easier to roam the black forest and when I'm attacked by skeletons I know a burial grounds is near. Just raid that to get the few surtling cores you need. As you gear up and level up your blunt weapons, skellies are easy to kill and take their surtling cores.