Well for weddings, no matter what age should be a lovely dress. Now it’s perfectly acceptable to buy a dress from anywhere as long as it looks appropriate for a wedding. Now if you live in the US just look around in your local shopping mall, there’s bound to be hundreds of nice dresses. if you live in the UK, i recommend Topshop or River Island. Although these may be a little pricey then you might want to look around at Asos (an online store).
In my opinion the dress should either be plain with lots of nice accessories or floral with minimal accessories. Shoes: heels or gorgeous sandals.
Hope that helped 🙂