Where to get signet of capture

Forgive the newbie question. I see all the talk of the Signet of Capture, but where do I get it?

And is there a list of skills you can capture from what boss, and where the bosses are?

I really want the Ranger trap skills, but don't know where they are.

You have to do a quest in Lion's Arch in order to get the Signet of Capture (aka, SoC). Think an NPC named Magi or something gives it. After you get it, you can buy a new one at any skill NPC since using it means you lose it.

Various sites are trying to gather a list of all the bosses that give certain skills, but I don't have any on hand (you can probably use the search feature on these forums to find a list here).

The signet doesn't get lost and you do not have to buy it.
You just have to re-equip it back in town after you successfully capture a new skill.
The signet gets given to you...just like a ressurection signet etc

No, that is incorrect.
When you use the signet, it is replaced with the captured skill PERMANENTLY, so you can not use the capture signet until you buy it again.
However, buying capture signet only costs half as much as buying other skills! So even if you use capture signet to get skills you can already train anyway, you will save money by doing it.