Hi James,
You can't lose with any of the recommendations you have been given but here is something to think about. I have bought 5 flags from Ben Tart and he does an excellent job but sometimes he gets backed up and it could be awhile before you get it. He also uses a fabric that is could pass for wool bunting which is what you want if you want greater accuracy. It has the feel and texture of wool bunting and is not heavy like cotton. However, I believe he only does Confederate flags (that is all you see on his website). I have bought flags from both Tom and Richard and they also do a wonderful job, very accurate and they give good service. But from what I understand and they only make flags in nylon and cotton. I am not sure about Steve. While some flags were made of cotton during this period most of the government issued flags were wool bunting for the CS and silk for regimental US flags and wool bunting for the US national flags, like garrison flags, naval flags, etc. (Some US national flags were made out of silk too). In my opinion since you will paying extra for a custom flag I would prefer the fabric that looks and feels like the real thing. Just my humble opinion. Like I said you are in a win-win situation. All these guys are great!
Cheers,
mike