Before going shopping do some research, look online to see what you may like and what may be available.
I would suggest initially having a discussion with the bride, and the other mother to find out their ideas about formality, styling ,themes and color. Keep the brides thoughts in mind, but ultimately you will be wearing this outfit and you need to feel comfortable and be happy.
It is a good idea to buy an outfit that will sit for both the wedding ceremony and later into the night for the reception. Unless you intend to change into another outfit, it is a good option to purchase a dress and jacket. Wear the jacket over the dress for the ceremony and then take the jacket off for a fresh look into the evening.
Consider the time of year
Depending on where the wedding is being held and what time of year, the weather will play a significant part in what outfit you choose. If you live where the climate can change at the drop of a hat, you may need that jacket just in case, even in summer.
It can be tempting to buy a size smaller , thinking that you will lose that extra few kilos, but why put yourself through that extra pressure. If you want to lose weight, buy a dress that fits at the time it can be altered closer to the wedding, but it will not normally be possible to let out.