Found with the Brigand Armour set.
This is found in The Gutter. Move through the zone until you cross a suspension bridge under another bridge. On the other side, bear left and look for some ledges you can drop to. Follow this path until you climb a series of a ladders to a zip-line. The pot underneath the line holds a corpse; bust through and loot the corpse for this weapon.
Black Knight Greataxe
To get this, first head to the second section of Iron Keep (with the walkways and greatbow-wielders). Take the ladder in the back, then follow the stairs to a second ladder. This leads you to the roof, where you can battle a trio of Alonne Knights and open a chest for this weapon.
Can be traded for the Soul of The Rotten with Ornifex near the Lower Brighstone Cove bonfire in Brighstone Cove Tseldora.
This is found in the Dragon Shrine. After heading up the steps to fight the second large knight, head up the steps under the archway on the left. From up here, you'll have a bridge you can take to the gazebo with the sorcerer enemy. Head up there, take him out, then take a running leap off the side of the platform to the nearby rooftop. The chest here contains the weapon.
Dragonslayer's Crescent Axe
This if found by the Ruined Fork Road bonfire in Shaded Woods. Take the leftmost path, and follow it. At the end, you'll have to face a weaker version of the Flexile Sentry boss. Take him down, then pick the corpse beyond for this axe.
Giant Stone Axe
Can be traded for the Soul of The Last Giant with Straid of Olaphis at the Straid's Cell bonfire in The Lost Bastille.
You'll find this by the Gyrm's Respite bonfire. Enter the large chamber full of Pharros' doors and look for an opening that leads uphill. This will take you up to a corpse you can loot for the axe.