Tempest Revenant can be found in Wintergrasp.
These revenants are only on the south and south-western edge of the zone.
Although the zone was extended further south in Patch 3.0.8 to reach more revenants, occasionally Tempest Revenants will be outside the Titan Relic's buff zone. This results in a revenant that can be seen, but cannot be touched, except for pet user classes. Pet classes can remain in the buff zone to keep the revenant visible, while their pet is sent out to aggro the mob and drag it back into the buff zone.
Be aware of where the 'border' is, or risk killing a revenant, only to run up to loot it and have it disappear on you.