You can buy a roll of clear plastic and carve off a suitable piece. You can tie ropes to the corners with a sheetbend
http://www.folsoms.net/knots/
(The rope needs to do the work. The plastic just froms the "J" the knot is tied into.)
I'd recommend thomas' suggestion above to try out tarping. Go to home depot or some place similar and buy some 3 mil polyethyline plastic. 3 mil is a nice thickness since it's strong and thick enough while still being light and packable. You can use the same material for your groundsheet. 8x10 is a good tarp size for 2, 7x9 is good for solo use. Use some paracord tied between two trees for your ridge line, or you can also use sticks at the ends if there are not suitable trees. at the corners attach some 4 foot or so guylines with sheet bends as suggested above. The sheet bends hold surprisingly well, I've never had one come undone.