Useful Armour

Daedric:
Boots Galom Daeus, Dwemer vampire lair, far N of Suran, in the volcanic region.
  Cuirass Telvanni Quest, Mistress Therana, Tel Branora, but enchanted with 50 pt feather.
  Helm  Face of Inspiration: Ibar-Dad, W of Urshilaku Camp.
        Mamaea, W of Berandas.
        Galdal Omayn, Tower of Dusk, Ghostgate
    Face of God:  Sepulcher, Tukushapal, Marvani Ancestral Tomb, S tip of Molag Mar peninsular.
    Face of Terror:  Enar Dralor in Lower Level of Tower of Dusk, Ghostgate
        Forge of Hilbongard, Forgotten Vaults of Anudnabia, Omaren Ancestral Tomb, island E of Sadrith Mora
  Greaves Dren Plantation, on a shelf behind Orvas Dren.
    Druscashti, W of Khuul, over the mountains.
    Telvanni Quest, Mistress Therana, Tel Branora, but enchanted with 50 pt feather.
  Pauldrons None available, but see full set below.
  Gauntlets Actually bracers, Kogoruhn, unnamed Dunmer Stronghold, Bleeding Heart area
  Shields Galom Daeus, Observatory (see boots)
    Galos Farethi, Dren's Villa, Dren Plantation
    As random loot on Golden Saints. This is also the only way to obtain a daedric tower shield.
    Azura's Servant, an enchanted daedric shield, in the NE tower, Tel Vos.
Full Set Minus Helm: Divayth Fyr, at the top of his tower in Tel Fyr (levitate required), has a full set of daedric armour, apart from a helm, including the pauldrons which are unavailable elsewhere. There are two/three ways to get his armour:

1:) Just kill him for them. Wait until you have completed the cure corpus section of the main quest before doing so, or you'll never be able to complete the game.
2:) This method requires that you have not installed the second patch, and also works on X-Box. You need a disintegrate armour spell, a charm spell, a calm spell, and a very good hand to hand skill. Disintegrate armour and run away until he is naked. Now pound him with fists until he is unconscious. Take armour. Wait until conscious, then calm and charm until he is friendly again Without the patch, you can use this to strip anybody of their unequipped items.
3:) This works with the patch. As above, but after disintegrating the armour, simply pickpocket it from him. Okay, it's not 'simple', but it works if your skills are up to it.

Non-Daedric:

Saint's Shield: An ebony shield with feather upon use, easily bought for about 10000 from the temple in Balmora.

Glass Armour: Go to the Ald-Ruhn Fighters' Guild. Downstairs is a level eighty locked door, behind which lies two chests. One is unlocked and contains boots, greaves, cuirass and helm. The other has lock level 100 and contains full orcish armour. You actually get the key for these doors as a quest reward from the Fighters' Guild. Also go to Ghostgate, where glass armour can be bought, stolen from shopkeepers, or stolen from the vaults. The Great House vaults also contain some glass items. Most of the pieces are common, but the bracers can only be found in the Tower Of Dusk Lower Level at Ghostgate.

Indoril: Full suits of Indoril armour can be stolen off any ordinator, or found in a number of locations. Look in Ghostgate for some pieces.

Dreugh: Only a few pieces of Dreugh Armour exist: cuirass, helm and shield. The cuirass can be found in the Trader in Suran. The Shield and Helm are easily 'borrowed' from Meldor's in Balmora.