Quest ID 8414 "Dispelling Evil"
 

The Scourgestones you need for the quest are dropped by the lower level undead in the graveyards around 50.0, 79.5 in Western Plaguelands.

Seems to be found on any undead in Western Plaguelands, as wherever I go I seem to be collecting them. The undead at the Ruins of Andorhal and the Farms north of it also drop Scourgestones.