I buy a lot of good labels through vintage or recycle boutiques, lots of great, timeless designs and quality fabrics. So that may be a worthwhile idea if you have one in your area.
One thing I have noticed when reading the replies to this thread is that it seems some people's family members are not backward in coming forward regarding telling others what to wear. Since when did the matriachs of any family need dressing tips from young ones or the in-laws? One of the fabulous things about being a grown-up is with maturity you develop your own sense of style and colour. If we love our children enough to attend (and usually pay for) their weddings, should they not respect their parents clothing choices?
Sorry for the rant...but honestly, this is a quilting site, quilts are art, colour, design...surely those things are also reflected in other elements of quilters lives?
Wear what suits you, what you feel comfortable in and by no means does that mean "unglamorous" nor should you dress down for anyone. The bride always shines...it is her special day, so just relax and happy shopping.