SPOILER! If you want to figure it out on your own, don't read this trash. This is pretty well a full walkthrough of it.
*** This is obtained from the bait and switch quest in Bastok MetalWorks. Talk to the Fat NPC on the second level, just near the elevator. It will start a quest in which you get to pick from a number of items (Choose Lucky Charm to get the belt) and then you have to go hit 3 different switches a certain number of times and in the right order. The number of times and the order can be found by talking to various NPCs in the metalworks (the kid galka, the old man, the guy near the old man, and the galka in the restaurant/cafateria thingy). The three switches are 1. the big kiln 2. the bookcase in the cermet refinery 3. the map of bastok right when you enter the metalworks. When you try and press a switch it will bring up a CS and a guard tries to stop you. If the guard was talking about the thief, let the mithra handle it, if the guard wants to go have a pint, let the taru handle it. When the boss guard decides to intervene, you'll have to figure that one yourself (hint: make sure to look at all your options). Go back to the place where you met the pair of thieves and claim your reward! ***
I've done this quest once before but only got a Silent Oil from it. When I was looking for NPC's for the Sentinal Badge quest in Bastok, I got it again. So yes, it is repeatable.
I saw a 50ish thief with this the other day, but she wouldn't tell me where she got it. All I got out of her was '[secret].' As if it won't soon be common knowledge anyway.