A little story before you order your dress. Once I went in for a bridesmaids fitting and the sample dress fit me almost perfectly, yet the lady insisted on ordering something two sizes larger. We had a heated discussion, and she finally gave in ordering the correct size and I only needed a little taken off the length. Some places will purposely order dresses way too big to get extra money for alterations.
I use Hanna Cleaners and they have done alterations on my daughters prom dresses for a reasonable price. Check out the local dry cleaners for their hourly rates. Many have seamstresses.
Good luck!