Try Gordmans, Kohls, Jcpenney, Carsons or the mall. I know don't what stores are near you but these are good for a casual dress. Just try not to outdo the bride. A pretty shade in the blue spectrum maybe. Nothing too long for a wedding as you might want to dance.
h&m or macys
It is difficult to tell anything particular without knowing your budget and the city.
Short notice! If casual it makes life easier, any high street will have summer style dresses/skirts. You dont mention your age or budget to narrow it down.
