Before I realized I was broke and had no money, I was seriously considering dressing up as a Starfleet officer for Halloween (Last year it was a member of Stargate SG-1). I was inspired even more after going to Star Trek the experience. I did quite a bit of research and found some helpful sites and links. 
I looked on E-bay initially, but didn’t like what I saw. A lot of the costumes were simple jumpsuits that looked really cheap. I think the manufacturer of my least favorite costumes were by Rubies.
I want the real pips & pins, including an authentic looking com badge. I want to get a TNG Season 3+ uniform. I decided the only way to do this was to make one myself. I found a couple of sites where you can get the pips & pins and a pattern to make a nice looking uniform.
You can get your pips, pins & badges here:
You can get your patterns here:
Roddenbery.com
Exscapeprops.com
Starland.com
Please be aware that I have never actually purchased from these sites, so I cannot verify their credibility.
Enjoy making your Star Trek costume this year!