According to
the wiki Torpor can drop from any "Kunark Level 50+ Mob". I'm not sure that's correct though; I thought it only dropped from level 60+ Kunark mobs (if someone can confirm that I'll happily fix the wiki).
Regardless, common sources include the Emperor (at CE camp in Sebilis), Kunark raid dragons, VP entrance Guardian Wurms, Clay Guardian (protects OT) and Sebilite Protectors in Sebilis.