You get the Key to the Palace of Lei Shen from rares and troves on the Isle of Thunder. They drop from other mobs too but the chance is really low. Ive also had them drop from snails in ToT.
You get Elder charms through the scenario that the key unlocks. You also get them from using the incantations of deng/vu/haqin and killing the summoned mob. The weekly quest only gives Mogu Runes now so there is no way to convert lesser charms into greater ones.