SPOILER
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amulet of power - Aran Linvail (Head of the shadowthieves, 3rd chapter)
boots of speed, gauntlets of weapon expertise - planar prison
cloak of mirroring - sahuagin city (4th chapter)
staff of magi - unmarked house in bridge district (north of kidnapper's lair I think), need a rogue stone to gain access, very tough fight.
You can buy one girdle of giant strength at the adventurer's mart (forget which one)
Robe of Vecna is only available if you have the "bonus CD" installed. If so, two merchants will appear (Deirdre - Adventurer's mart, Joluv -Copper Coronet). One of them sells the robe.
The files of this CD are available at
http://members.home.net/mrkevvy
One of this bonus merchants sells a very special katana, try it.