Starting with the Death Knight quests, the Argent Crusade were formed from a union between the Argent Dawn (a Vanilla WoW faction) and the Knights of the Silver Hand (an order of Paladins that were reformed by Tirion Fordring)
The faction vendor for the Argent Crusade can be located in the Argent Vanguard in Icecrown. He sells a lot of plate items so Paladins rejoice, but you do need the required rep to buy items, so if you are short on rep, you can purchase the faction tabard and grind some instances!
