Dress Wanted- Maggie Sottero Sarchi

I have officially fallen in love with the Maggie Sottero Sarchi dress. My only problem is I`m not sure what size to buy... I normally wear a sz 4-6 in dresses, my waist measures 25.5 inches. I looking for the all white dress but it`s $1,000.00 at the shops around my home, I just can`t afford to pay full price! I would very much appreciate your advice on a size and how I can buy one for less that full price.