The Reins of the Thundering Cobalt Cloud Serpent are a very rare drop from Nalak <The Storm Lord>, a world boss cloud serpent on the Isle of Thunder. Nalak is made available once the realm has completed The Fall of Shan Bu portion of the Isle of Thunder storyline.
See Nalak's article for abilities and tactics to defeat him.
This is a descendant of the irascible Nalak. Lightning erupts from its scales and arcs over the saddle. Dare you sit?