Sunfury Signets are carried by high-ranking followers of Kael'thas, although Illidari blood elves carry them as well. They can be collected and turned in to Scryers for reputation. Each item gives 25 reputation all the way up to exalted. The Aldor equivalent is the Mark of Sargeras.
As a quest objective Edit
There are many places that can be farmed for Sunfury Signets. The various Manaforges of Netherstorm are crawling with Blood elves, as are Eclipse Point and the Ruins of Karabor in Shadowmoon Valley. Tempest Keep has a high drop rate, but there are obvious difficulties with farming an instance. Almost all of these areas are large enough to support many players at the same time. Thus, competition with other players is almost never a problem unless one faction starts PvPing with the other.
Because of the great variety of farming locations, there is no single best place to farm. Try them all and find what area best suits your own class and talent spec. With the exception of Tempest Keep, drop rates are fairly uniform. Look for an area that allows you to kill the most mobs in the shortest amount of time.