First off, today McQueen is releasing the Pippa dress on Net-A-Porter for 1,995GBP. Find it here! There are some minor differences between Pippa's dress and the one shown online (the finish on the neckline and sleeves), but otherwise this is THE DRESS.
Jim Hjelm also has a bridesmaid dress available - in both white and colors (the blue is shown below). I love the blue that they use, and you can also pick and choose the color of the lace as well. Plus I love that it is different enough from Pippa's dress to make it your own, but at the same time has a classic style that's fit for any venue.
Debenhams also has 2 knock offs - Pippa's ceremony AND reception dress! I'm personally not a fan since you can tell the difference in quality and construction (look at the comparison below)... but they are available.
As I said - the great thing about the style is that it is classic - but my recommendation is to make it your own! Pippa looked AMAZING that day (future hubby's eyebrows were raised), so compete with Pippa at your own risk!