You might also try making your own tarp. Ray Jardine gives extensive instructions in his "Beyond Backpacking" and "Ray-Way Tarp Book," the latter being geared towards the kits he sells to make the job easier. Jardine's designs are excellent (I subscribed to the Golite e-mail list and won one of theirs some time ago. They're a bit overpriced if you buy them new) but the features that make them so suitable as ultralight tent replecements also make them a little less versatile than the simple, sheet-like traditional tarps, such as the Aussie and British army models. Making a simple sheet tarp yourself obviously allows you to tailor materials, colour/pattern and attachment points precisely, and the amount and complexity of the sewing is suitable even for the novice.
A quick search for materials should point you to suppliers of seconds-grade fabrics, too, which often have only cosmetic flaws and keep the cost right down for top-spec stuff.
Even if you don't fancy doing it yourself, or recruiting a friendly machinist to do the sewing for you, I would certainly suggest taking a look at some of the ultralight hiking pages on the 'net to give you an idea of the performance of different materials and types. If I recall, a British supplier of ultralight equipment, Hike-Light I believe (I'll see if I can find their site and post a link), are currently offering certain Golite models with free net tents, designed to keep the winged nasties away. Again, if you want to sew one yourself have a look at Jardine's books, although reviews suggest that his net tent attachment for the tarp he designed is a bit of a contortionist's job when it comes to getting inside.