The Regimental Quartermaster is the granddaddy of current Civil War tent sutlers. I've bought a few things from them over the years, and they seem to be fine fellows to do business with.
The Regimental Quartermaster is a one-stop shop for reenacting stuff. They have a great selection of items, but you have to be careful about what you buy if you're gunning for an authentic impression. RQ appeals mainly to the mainstream crowd, but they have been doing a great job of offering campaigner stuff, including a selection of defarbed rifles. If RQ has a strength, it's that they sell a wide variety of things that other sutlers don't carry. Impress 'em with size!
Their site is nice and uses up-to-date design, something few vendors sites do, but should do. Makes life much easier for the shoppin' reenactor. They do need to work on some of the links on the Campaigner pages -- it's hard to get back to the Home page, and the bottom link doesn't work.