PvP Bountys are of course found in PvP! You will find them in the loot chest ‘Reaper Supply Crate’ that you get when you win a PvP match.
Bountys do occasionally drop from players during the PvP matches but this is super rare.
There are 47 sets to find, each with a Foot, Hand, Eye, Head and Heart.
The bountys that drop from the Reapers Supply Chest are completely random, there is no way to make certain classes drop, the classes/souls of the players within the PvP match have no bearing on the bounty drop.
The Primalist Bountys are the most rare as these were added last.