Black Cocktail dress

Because I am on the heavy size and I am attending a wedding this coming April, a search for formal dresses for plus size women is in order. I have only one black formal dress which I wore to J-mie's wedding last November 2009. I can still wear that though but the lady in me is wanting to wear something like a cocktail dress and hopefully not black although black can make me look slimmer. But on a second thought a black cocktail dress I think is perfect.. like this one..

Now, the problem is, where can I find something like this here? and how much would it cost me?