I really think you should go for the bespoke option here to make sure that every bridesmaid (assuming there is more than one) looks right, am off the peg bridemaid dress for a girl so tall would not be a simple shopping trip! Also, make sure the shoes are low heeled or even flat, especially if there are other bridesmaids, it always looks better on the photos if they are all roughly the same height, so shorter hirls would need higher heels.
It isn't always false economy to have the dresses made, as they are far easier to sell on ebay, due to them being easily alterable and unique. Plys they will look well fitted and the right length!