It'll vary to some degree from chapter to chapter, but typically NPC camp will provide whatever costuming you need. Most chapters suggest you wear a dark base layer (black or brown) and they'll provide whatever you need to go on top of it. I personally bring a lot of my own costuming for plot duty at Chicago, but I use a lot of the chapter's costuming as well.
I would avoid wearing the exact same costume you would wear as your PC for any of your NPC roles, but generally it should be fairly easy to change up the costume just enough to make a notable distinction.