There is one way, and that is the Friend Safari. Prankster is the Hidden Ability of Sableye, means that it can't be obtained through normal means.
There is a chance that Pokemon from Friend Safaris may have also have a Hidden Ability, I'm positive that the Ghost Safaris have a Sableye. Go into a Safari that has a Sableye, and look for the right ability using the ability Trace or something of the sort.
Hope I helped!