I bought my daughter a beautiful white dress off eBay, it was a bit too small and we needed to make it a bit more conservative since my daughter is getting married in the LDS temple.
Bridalxoxo worked with us, put in a partial lace up corset back and some elegant draping to cover the backless portion and the tops of the shoulders. Looks perfect, and didn't break the bank. They will work with you until it's perfect!