The vendor - Deidre - in AM sells Robe of Vecna, which is indeed a very good caster robe, best in game. However, she does NOT sell Cloak of Balduran. She has Shield of Balduran, Plate of Balduran and Sword of Balduran, only. In the game is also the Helm of Balduran, at the beginnig in Jon's dungeon.
Neither the another "special" vendor in Copper Coronet sells the Cloak of Balduran. It's simply not in BG2, sadly. Or if it is, then it's damn well hidden..