Not trying to rain on anyone, but while RCJ has really nice clothing, it's not in everyone's budget.
Colt, if you are talking about the duster they show in brown on the website, it is fine for NCOWS. The only time it might be a problem would be if you were trying to get certified for the Originals category. Then you would have to be able to document that coat, with slant pockets and metal buttons, as being available for the time period you are going for. In other words, if you just want one like the one Texas Jack's has for shooting matches (including a National) in one of the standard categories (i.e. NOT Originals) that coat will be just fine. We really just are not that strict.
FYI, there were lots of pocket styles. Straight patch, flapped inset, slanted, flapped outer patch. And button material - metal, bone, antler, cloth covered metal (fancier frock coats), wood. I don't know when plastic buttons first started showing up, but if it was not in the 19th century, even going to River City Junction and paying $80 for a pair of trousers, is a problem. Besides, you can always replace buttons if you don't like them.