Depends on which one you're going for, Season 1/2 or Season 3. For Season 1 and the first half of Season 2, I think the material used was lightweight cotton velour. Halfway through Season 2, they changed it to double knit polyester. I'm sure both fabrics are available. For both uniforms, the color was avocado/mustard but when seen on film it looked gold. You can find more info here: http://www.startrekuniformguide.com/originals2.html
You need to check over on ASAP as a group is trying to get the right Third Season fabric.
The First two Seasons were Velour and as with all of Trek's Command Shirts a very hard Color to peg. That Velour is impossible to find now though
The Third Season is not double-knit like everyone thinks close though and is another weird Color, Theiss called it Avocado.
Sadly the people who DO know where to get the Fabric as well as where to get it Properly dyed, The Dye house who did it for the Series is still operating and has the exact formula, are no longer doing any Tunics or can not be trusted.
I have no time right now but maybe in the future, a year or two and no one
has stepped up to the Plate I will do a run of Fabric for Third Season Command
I'd try Roddenberry.com. The pattern is spot on. They have braid that seems pretty nice, but I've never seen it in person. It looks more like the bigger first season braid to me, but it should work just fine.