A basic one?

Like, I figured out that sleeks for one character differ from the sleeks to another. Are there multiple possible sleek locations? I heard that Bulkra's black was in a bad, bad place. Was that his only possible choice? If you find your black, is there no other possible source for it? Will your locations ever change on your character? And are the individualized sleeks just in containers or can they be on NPCs? I know for sure that there are unmatched sleeks on some area explore mobs, but can personalized ones be on NPCs as well? If a thief peeks at an NPC with a sleek and he's in your group will he see your sleek?

I'm thinking of rolling a mage, and I would like to at least know the basic mechanics, before I try anything.

Your set of three sleeks are your set determined when you roll your character, they do not change. There are plenty of wands out there to be an alternative to your set, some are even called a sleek but they are available to everyone, not just your character. Only you can see your own sleek spots, no one else will be able to see them unless it is also their spot. As to where they are, how they are placed on the ground on a mob etc. that's for you to figure out in game. As far as having one of your sleeks in a difficult spot, remember there's always alternatives.