I'll second tableclothfactory.com. They come with the edges hemmed so unless you want sewn channels they are ready to go. I've made one for each of my three kids using the 60" x 102" polyester taffeta. It's light and strong. If you decide to go this route I think you can put "hammockforums" in the gift code and get 10% off.

I've also used polyester fabric from Walmart. I got 4 yards and it's perfect for a single layer but I wish it was a little bit wider. It's starting to show wear as well so it's getting retired.