The string you don't get until chapter four, in Spellhold.
The gauntlets are in the planar sphere - Valygar's quest. Find him in Umar Hills.
The warhammer is in a small illithid lair hidden in the temple district sewers. You'll need a key from the Windspear Dungeon to get into it, don't ask me how it got there.
The belt you get in chapter five, in the Kuo Toan lair. There's an altar that asks for an animal sacrifice. If you oblige it and manage to kill the things that appear, you'll find the belt on one of their corpses.
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