Shop early and often. Sometimes by shopping off-season you can get quite a discount on wedding dresses, regardless of designer because they are trying to make room for the new fashions of the upcoming bridal season.
Buy a discounted or last season dress that is close to what you hope for, even if it is a little big. Simply have alterations done to change it. You can also add sequins, train, weights, and more to put the finishing touches on the dress that will be uniquely yours.
Visit vintage bridal shops and thrift stores. These often have several different wedding dresses in stock and ready to go at deeply discounted prices. Just make sure there are little or no defects.
Look online for a discount wedding dress. Always make sure that you can return it with no questions asked if you choose an online purchase. This will be more difficult, but it could result in a substantial discount on even a designer wedding dress.