I have two Star Trek costumes, a TWOK red uniform jacket and a klingon costume.
The Klingon costume has an authentic headpiece, but the seamstress/costumer who made it was creative with the rest. It looks good and it's won some contests, but it isn't anything close to accurate.
When I had my TWOK uniform made, I hired a seamstress who made it from patterns and instructions from Lincoln Enterprises. She got all the details down (like the little chain and a little piece of velcro to hold up the inside white flap when the outside red flap is down). With pins and insignia, it cost me $250 tailor made and the costume was a hit at the conventions. It's survived conventions, costume parties, and many trips to the dry cleaners (who always look at you funny when you request repair work on the little chain on the inside).