Below is a list of some well-known Civil War vendors and sutlers but it is far from being a thorough list. There are several more that are reputable and affordable. I highly recommend spending time looking at these sites and many others to see what fits your style and your budget.
Most of the resources listed sell not just the item pointed out, but several items you may prefer.
Two places to start with whenever planning a new dress or outfit are:
(undergarments, stockings, shoes, simple work dresses)
Fall Creek Sutlery
(Heather does very comfortable working corset)
The Paper Lady
(usually at events, but you can contact her for a fitting)
(bonnet patterns, ribbons, trimmings, etc.)
(lovely items! They are pricey, but great place to look for ideas)
(she also sells on Ebay)
(also most sutlers have patterns for sale and there are many ladies happy to point you in the right direction for patterns)