Look for any place in your area that handles a product called SHADE CLOTH. Greenhouse or Gardening supplies. There are a lot of places that sell it on the internet, but the most cost effective place will probably be from a local supplier. It usually comes on a giant roll like carpeting and you order it by the foot - comes in widths like 16 foot or 24 foot. I actually also found a local place that sold precut pieces 16 foot square - but only in 55%. It also comes in many different amounts of cover - approximately 30-70% - here is a site just to give you some ideas of some of the types that are available -
Shade Cloth