I've found a couple that give stats about items, but none seem to have any information of what monsters drop them.
[EDIT]This post is Wrong!!... see my post below for the correct one.
http://forums.overclockers.co.uk/showpost.php?p=12001472&postcount=385
Items are given a Quality Level (Qlvl), so if you wanted a Tarnhelm (Qlvl 20) you have to kill monsters that can drop Treasure Class level 20 (TC 20) or higher...
go to some site like the arreat summit to find the items you want:
http://www.battle.net/diablo2exp/items/uniques.shtml
and then search for the item on this page (Ctrl + F to make it easier):
http://www.diabloii.net/wiki/TC_1.10
remember the qlvl number next to the item name and go to:
http://www.diabloii.net/wiki/TC_SuperUniques_v1.10
if your looking for that tarnhelm look down the TC columns for anything with 20 or higher and they will be the boss monsters you can kill for that item
if you don't know where the monster is use the search at the top left and just put it's name in, you should get a page with details about them and where to find them.

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, Mlvl is the Monster level which is the same as the Qlvl (mlvl = qlvl) , if you want the Immortal King's Soul Cage item you have to be killing stuff that can drop TC 87 items that is Mlvl 37 or higher.
Doubtful you can trust anyone off battle.net either 

Managed to pick up the last copy in another store but no expansion.
, cant wait for Diablo III