Capture Signets

Where do you get a capture signet? I was told it was somewhere around Lion's Arch... and I finally got there today! But I don't see a Capture Signet at the trainer... Where do I get it?

*Bump*
I want to add on to my question too. I know that capture signet can only be used on boss creatures... but can it only capture elite skills? Or can I capture basic skills as well, as an alternative to questing for them (if the trainer didn't have the skill :P)

You have to do the Orsian Librarian quest or something. I forgot the exact name, but its the quest given to you from a person standing at the top of a hill in Lion's Arch. He asks you to go into th North Kryta Province to get him 5 artifact tablets. Once you complete that quest he'll give you the capture signet and a quest to capture a skill with it.