Watch out, spoiler ahead, (man I am generous with them today)
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Robe of good archmagi is sold by blacksmith in Trademeet, he is in upper corner of Trademeet map, left of Mazzy's house, right of Alibakkar estate (Don't ask me, how comes, that the only robe of good archmagi for sale in whole Amn can be bought from a blacksmith.
)