It's a spell available to both arcane and divine casters at different levels. For more detail check
this page and look at the left hand side bar where it says "Spells."
Sometimes you can also find one-use scrolls lying around.
Spells and scrolls are not that readily available in this game though, so you may be out of luck. You don't absolutely need PfL to win the battle with the Wisps, just some magical weapons (which you should have by now) and good planning. I also suggest you take a look at the inventory stock of the merchants in the Wandering Village, there might be something of help to you there.