Go to the local fabric store and look for some black duck. Ask, they'll know what you mean. It will probably be 36, 42, 45 or 48 inches wide, just buy a square. Ideally, you'll want to get the two loose edges seamed (if you can't do this, ask the sales person -- I'm sure they can suggest a customer who'd be willing to do it), but even that's not essential right off. If you want to get fancy, have some drapery weight sewn into the seams -- that will help keep it from blowing around on you.
White on one side is a nice option, but it is NOT essential. Once you've shot a bit, you won't be spending all that much time under the cloth, anyway.
Ed