Confirmed from Joll Darigon on Sunrunner Server
The Exemplar's Mark is dropped by the Exemplar Zheus (Outcast) that wanders behind Mysses Glade in the Dead Forest.
But sadly, the Mark is on a rare drop rate, killing him more than 27times will get the player along with Outcast loot.
Note: Along with the Mark, A Corestrike Rancor drops from him aswell much common than the Mark(in some cases)