Use a bridesmaid dress rental service
How does it work? Check out the dress collection on a rental site like Union Station and choose the color and style you like. The dresses are designed to flatter many, many different body types, so costly alterations are not necessary. Once you put your order in, Union Station will send you the dress two weeks before the wedding date. They send a pre-paid envelope, so you can just drop it in the mail after the wedding.
[Photo via Union Station]