Put this together for my guild, thought I'd share. Figured people could print this out or something if they are trying to gather old tier gear for next patch.
TIER 1 – Molten Core
• Waist and Wrists – BoE – Molten Core Trash
• Chest – Golemagg
• Shoulders – Baron Geddon
• Helm – Garr
• Legs – Magmadar
• Hands – Shazzrah
• Feet - Lucifron
*Check your class on Tier 1, some stuff drops off other bosses depending on your class
TIER 2 – Blackwing Lair / Molten Core
• Helm and Chest – Nefarian - Blackwing Lair (Can get a Helm replica off Onyxia from the Wrath update)
• Gloves - Firemay, Flamegor and Ebonroc - Blackwing Lair
• Bracers - Razorgore the Untamed - Blackwing Lair
• Belt - Vaelastrasz the Corrupt - Blackwing Lair
• Shoulders – Chromaggus - Blackwing Lair
• Boots – Broodlord Lashlayer - Blackwing Lair
• Legs – Ragnaros – Molten Core
TIER 2.5 – AQ40
• Wrist and Shoulders - Princess Huhuran or Viscidus
• Head - Emperor Vek'lor or Vek'nilash
• Legs - Ouro (optional boss)
• Chest - C'thun
TIER 3 – Vanilla Naxxramas / No longer obtainable
TIER 4 – Karazhan, Gruul’s Lair, Magtheridon’s Lair
• Helm - Prince Malchezaar - Karazhan
• Gloves - The Curator – Karazhan
• Pants - Gruul the Dragonkiller – Gruul’s Lair
• Shoulders - High King Maulgar – Gruul’s Lair
• Chest - Magtheridon – Magtheridon’s Lair
TIER 5 – SSC, TK
• Helm - Lady Vashj - SSC
• Gloves - Leotheras the Blind - SSC
• Pants - Fathom-Lord Karathress - SSC
• Chest - Kael'thas Sunstrider - TK
• Shoulders - Void Reaver - TK
TIER 6 – Hyjal, Black Temple, Sunwell
• Gloves - Azgalor - Hyjal
• Helm - Archimonde – Hyjal
• Shoulders - Mother Shahraz – Black Temple
• Pants - Illidari Council – Black Temple
• Chest - Illidan Stormrage – Black Temple
• Bracers - Kalecgos and from the Eredar Twins - Sunwell
• Belt - Brutallus and from the Eredar Twins - Sunwell
• Boots - Felmyst and from the Eredar Twins - Sunwell