This collection of armor is noteworthy for being the strongest mundane armor in the game. In many cases, it's the best armor available for a particular slot until you're well into your adventures. The only disadvantage to Plate Armor is its high encumbrance values.
All pieces of this set can drop among loot, although the probability is very low.
For AG characters, it can be purchased at Gryphook's in Tryndmoor (very expensive, more than twice "actual" value).
Breastplate: 28,150 gold
Helm: 22,500 gold
Sleeves: 22,500 gold
Greaves: 22,500 gold
Gauntlets: 11,250 gold
Boots: 11,250 gold
Belt: 11,250 gold
Non-AG characters will have to grind for it, but it's worth the effort, especially since non-AG character don't have access to many armor pieces available as quest rewards.