You can start looking for patterns on the Simplicity or McCall's websites. Much of their current catalogs are online.
If you don't have much experience with using patterns, reading this thread first may help: http://www.cosplay.com/showthread.php?t=242887
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