This dropped of the Enraged Soulstealer in Dulak...got my Soulstripper and this page...sold the page in bazaar for 2500p in less than one day...And I have killed the enraged soulstealer atlast 10 times helping others get the soulstripper and cloak drops ONLY once did this page drop so I would say it is rare...but it did drop it once so I am sure it will again...
This dropped off of a luggald abomination or initiate in Nadox for our group. I speculate it could drop off of anything in Nadox really, just really rarely.