If you're looking for gown and dresses for event, you can try Kenanga/KWC. There's a shop on Ground flr(if not wrong), that sells bride's gown, dinner dresses and even belly dance's clothing. the last month i saw it, some dress got size up to 4-5xl. price won't be cheap though.

or, you can try mark & spencer, debenham and metrojaya, blook+(i remember one at Subang Parade), Girlie's Barn(blogshop with physical shop at Subang Parade too) and also this Jessabella Plus size with physical shop at Shah Alam(closed on Sunday).