I picked up a black Diveskin online from some Divers Supply place in Florida. These have no thermal properties and have an SPF 50 rating, but can seem rather undersized, so check customer comments and order accordingly. Using my own experience: I require a large but bought the 3X based on the comments posted, it fits well but answering nature's call could be a challenge.
The slick nature of the fabric allows your fursuit to glide over you without rubbing your skin raw.
A black union suit, if you can find one, also works if you don't plan to strip off your fursuit in a public place--or just wear a t-shirt and gym shorts to hide the button front and seat flap. Plus you don't have to worry about a shirt coming untucked as you move about. Jockey was selling black ones in women's sleepwear IIRC.
As for balaclavas. It's a must with certain fursuit heads, to keep face and scalp from irritation. They sell cotton ones on Ebay. There are cotton-blend ones in a number of colors at Sportman's Guide, also military surplus ones. They may have other items adaptable to fursuiting or cosplay.