Dungeon set 5 items drop randomly (rarely) around the world starting on level 75 enemies and up until level 79. They also drop more frequently off trash mobs and assuredly from bosses in the Daily Quest dungeons: Imirian Ravine, Frost Swamp, Oasis of Zaara

Sets contain 6 items. The Wrist, Feet and Belt are all tradable. The Chest, Shoulder and Head are not.

Dungeon Set 5 (level 75-79)
Assassin	Barbarian	Bear Shaman	Conqueror	Dark Templar	Demonologist
Darkshroud	Berserker's	Rendclaw	Battlemaster's	Baleful	Nadiral
Guardian	Herald of Xotli	Necromancer	Priest of Mitra	Ranger	Tempest of Set
Dauntless	Rageflame	Soulbinder	Exalted	Ravenwing	Skyfury

Armor Sets
Class Sets	-     -     -     -     -     -     -     -     -     -     -
Dungeon (Group)	Dungeon Set 1 - Dungeon Set 2 - Dungeon Set 3 - Dungeon Set 4 - Dungeon Set 5 - Dungeon Set 6
Tier (Raid)	Tier 1 - Tier 2