i have a mother of the bride boutique in Edinburgh. the sort of thing you are describing could be Coterie or Anne Balon...the problem is that floaty things are classed as spring/summer stock and most of us are now selling autumn/winter stock. Our spring/summer arrived last november and most shops are the same...Anne Balon is light all year round and Coterie do floaty styles for all seasons too, so they may be what you are looking for.
Look in magazines like Bride or in scotland we have The Scottish Wedding Directory which is fantastic...i think it could be on-line.
hope your mum finds something xx