i just cant find one...

They are all over the place, actually. Look for an oval-shaped stone with lots of swirling colors... almost as if it were tye-dyed. If you really can't find any, then I believe the merchant inside the Shadow Thief guildhall sells one.

You can go upstairs in Mae'Var's guildhall and lockpick the vault-like door in the top right of the room. It has a rogue stone in it. Not sure if you can do it if you own the thieves' guild, though.

In my game after i did the Trademeet quests the guildmaster gave me one.