These merchans sell the Dungeon Vendor Armor Sets. These gear sets are designed specifically to help you progress through the Expert Dungeons. The sets for both faction share names and armor models.
The Defiant Dungeon Armor Set Vendors can be found in the Catari Command Center in the right underground wing of Meridian, around 6104, 913, 5081.
The Guardian Dungeon Armor Set Vendors can be found in the Lodge of the Plains, around 7594, 869, 3100.