I think they can spawn anywhere in Undershard, except for the assassins area. Last time I checked, I was unable to get one to spawn, however.
You might need to hang out for awhile, or bring friends to help bump up the spawn. In any event, it is probably not worth seeking them out specifically, unless you are looking for skins/bones.
The're some part of the map where stalkers spawn more reliably, near the Spire IIRC. They still mix with the other creeps though, I wouldn't recommend hunting them if you're looking for stalkers specifically.
