The best fabric to use for these uniforms is technically a 25oz felted wool called Melton wool. It does get beastly expensive, but it looks fantastic. If you don't want to spend that much, the most accurate is any heavy wool, or felted wool. If any wool is too expensive, then do as was mentioned above and get a bottomweight. They're thick and heavy enough to emulate the drape of wool, but nothing else can get the same nap or exact drape. Jo-Ann's sells a blue wool suiting that I briefly considered. It's $25 dollars a yard normally, but you can wait for a 40 or 50% off coupon. It's much lighter than the 25 ounce, and it's not as felted, but it still has that lovely drape of wool ~might be wool-obsessed~, and it's cheaper.
Here are some photos: A group shot
(Scar's hair was de-curled by Saturday, but we ran out of time), and a close-up
of Mustang's jacket. We've not processed our photoshoot pics yet, sadly.