For T'Challa: There are some adult-size Black Panther costumes floating around, but you can also DIY by wearing a long-sleeved black shirt and black pants and adding the silver designs around the neckline and forearms yourself.
For Nakia: Nakia is always in shades of green, so go with an olive-toned dress that has a high neck, cutouts, and long sleeves. Also bring along a gold clutch full of secret spy trinkets from Wakanda. With the sequel on the way, this costume couldn't be more perfect.