Getting Skills from Bosses

I haven't played GW for quite a while and I just started again. I remember that I had a skill / spell that would allow me to get certain skills from Bosses that I killed. I don't seem to have that skill anymore. So is that still an option, or where do I get that skill / spell. Thank you.

Signet of Capture, you buy them from skill traders:

http://wiki.guildwars.com/wiki/Signet_of_capture

Once you use it, it is permanently replaced with the new skill and you'll need to buy another. This is why it seems to have disappeared for you. See the above link to see where to buy new ones (I recommend buying 3-4 at a time so that you'll always have one when you need it).

Also, because it is not linked to an attribute, it will be on the bottom of the skill list if looking at your own skills or that of a skill trainer's.

And lastly; all other skills you can buy from trainers, only Elite Skills you have to capture. Unless you use an elite tome, a 1 time usable item that lets you choose an (unlocked) elite skill of the corresponding proffesion (of the tome).

Thank you all for your help, I am redoing the quest in Lion's Arch. Much appreciated.