I've got about 30 yds. of Silnylon bought at Walmart $1 table.
Just look around or put out a WTB on this and other outdoor websites. For my first homemade hammock, I used plain old ripstop, uncoated, full width off the bolt, and it's still going strong. I've used plastic sheet, plastic blue tarps, coated ripstop, and finally silnylon for flies, all worked well but 1.3 oz. silnylon wins in the weight department.
See if there is a Girl Scout store in your area and check out their bug nets. Much lighter....it's simply cheap netting with smaller holes. I've found it in all fabric places. Gotta say, your rain fly looks pretty slick in 'digital camo'!
Your project will morph into exactly what works best for you...part of the fun!