If you have enough marks and medallions you can just get one from the skirmish camp Legendary Items vendor. I think it costs about 3,552 marks + 404 medallions, or alternatively 2,664 marks and 942 medallions. Other than that they might drop from raids (the level-usability varies depending on the level) and 12-man skirmishes. For Dúrchest and the Twins of Fire and Shadow it can only be gotten if you do the challenge. The Silver Box of Sutcrofts Spoils can also grant you one.
[center][i]Do not trouble trouble until trouble troubles you.[/i][/center]
[center][color=green][i]Easca could manipulate a bull through a mousehole and still make it think it did it all by itself.[/i][/color][/center]