Return of Reckoning

Where do these drop?

champions drop them when scavenged sometimes

They drop from scavenging.

Armour fragments are scavenged from mobs of all levels.
In line with normal drop rules, Champion and Hero level creatures have increased chance to obtain Blue and Purple level fragments.

From my experience champs are the best way to go, hero's usually arent worth the extra effort for scavenging.