Where do you find the man that cursed Jaheira? I can't find any clues although the journal says something about the east slums.

Spoiler.
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You have three options...
1) Go to the slums and enter the building labelled "decrepit house" or something along those lines. It should be just north of the Slave Traders' boat. There you will face Ployer and his three mage cronies.

2) Go to the slums and talk to Bernard. He'll tell you that another harper (Belgrade) had the same affliction Jaheira does. Then you can follow step 1 or step 3.

3) Go to the government district and talk to the Cowled Wizard dressed in blue whose name is Corneil. Ask him anything about the other mages. Then head outside. Terrace will appear and offer his "services." He will either abandon Ployer or kill Ployer for you, depending on your choice. Then go to Ployer's home in the Slums and he will be alone. Either the mages will kill him or you can.

Grab the lock of Jaheira's hair and rest in an inn.