READ ME FIRST: Pattern Tutorial
Two most common commercial pattern companies' online catalogs:
Look through until you find something close to what you need. Since you don't want to do many alterations, focus on finding a dress that has similar seam-lines to what you want to make, while still looking for something in the same style as what you want. Skirt lengths can easily be lengthened or shortened, so don't use that as a critical factor in selecting a pattern. You may need to buy two patterns - one for the main body of the dress and one for the sleeves, for example.
Pictures and tutorials from my costuming portfolio at Syagria's Cosplay Site!
Star Trek Original Series Uniform!
Original Creation for AB13
Most Recent Con:
PAX East, Anime Boston 2013