Previous to when it was possible to research this excellent nuke, the mob we farmed for it was the infamous Thought Horror Overfiend.
The THO is a reasonably difficult raid encounter, and he may be found in the far southwestern quadrant of The Deep up on Luclin. There are several strategies for defeating him, and if anyone asks, I will post them. It si not necessary for everyone to be level 65 ubers however, since he was written at a time when lvl 60 was max. He is "perma-rooted" much the same as the Emp is (he can not chase you), and he has several attacks. The two attacks which give most players trouble would be his AE's... one of them causes casters to lose mana and fizzle constantly, the other essentially gimps your melee classes. The THO is a sort of hybrid cross between a warrior and an enchanter, and he hits fairly hard. Fun fight, however... just be prepared.
To my knowledge, the THO still drops this spell regularly, where as the Rune of Zephyr used to create Elnerick's Rendering is apparently an incredibly rare drop, and from mobs that could possibly be even tougher than the THO is. So, you may wish to simply set up a raid, and head to The Deep for a bit of fun.
~ Kj