I've always found it odd that mages have to kill other mages to get alot of the sleeks. Sure, if your light and the npc is dark or vice versa it makes it easy to RP. But there are multiple cases where it makes for a terrible RP reason.

Also, some locations (at least in the past) are impossible for some mages to be in and appropriate form for.

My suggestion then is to implement a quest option. Once a sleek holder has been identified via detect artifact the npc in question would offer a quest to kill something else appropriate and bring the object back.

NPC: "Ahh, you have noticed my mastery of the higher protective magics. Rather than contest me for it I will give it to you in exchange for this rare ingredient."

The quest would then be relative in power to defeating this particular mage and serves as an RP device rather than deterrent.

Thanks and Happy Holidays