Best place to Farm Volatile Fire is in Humboldt Conflagration killing Unbound Emberfiends just above Thundermar in the Twilight Highlands
Best place to Farm Volatile Water is in the Verrall Delta killing the Muddied Water Elementals between Firebeards Patrol Dragonmaw Port
Best place to Farm Embersilk Cloth is on the Twilight Shore between Highbank and Dragonmaw Port – killing Axebite Marines ( for Alliance – assuming for horde it would be Highbank Marines.
I like that they are all near each other – the fires and waters are still selling for a bit on the AH, and they have been good for crafting
I found that the respawn rates for the Fire and Water are ok as you just move around the zones.
The Marines for the cloth respawn constantly
There might be better places, but these are the areas I found to be the most convenient, with a decent drop rate, and because the initial questing frenzy seems to have calmed down the zones are not populated often – so mobs are free and clear.