Camels are available as a new ground mounts in World of Warcraft: Cataclysm. Obtaining one will award the achievement [One Hump or Two?].
Camels Requiring Faction Reputation
The following camels are sold by Blacksmith Abasi <Ramkahen Quartermaster> for 100, provided that you are Exalted with the Ramkahen:
Camels from Rare Mobs
The Grey Riding Camel is a guaranteed drop from Dormus the Camel-Hoarder:
Trading Card Game
The White Riding Camel is available from the Tomb of the Forgotten TCG expansion set, redeemed with Landro Longshot in Booty Bay.
Patch 4.0.3a (2010-11-23): Reins of the Brown, Tan and Grey Riding Camels added.