Lake Valor can be found in Arid Highlands biomes. You can also combine the Valor Proof (found in the Team Galactic HQ after beating Mars) with an empty map in a regional cartography table will look like this:

The largest island near the middle contains the Azelf Altar and a Pokemon Trial Spawner. Here a level 78 Overqwil will spawn. Upon defeating it, a Azelf Fang will drop from the Trial Spawner. This trial spawner is re-doable, though I'm not sure what the cooldown is.

Upon using the Fang on the Azelf Altar, a level 70 Azelf will spawn. An Willpower Jewel will also come up out of the Azelf Altar.