+1 on RSBTR. Kyle runs a good cottage distributor for home DIY folks.
I've gotten materials from Dutch, too and have been very happy.
Standard 1.1oz silnylon will be the best and most available light waterproof material out there. Silpoly is similar but with much less stretch when wet. Like Portagekeeper said, these silicone impregnated fabrics are extremely waterproof, but do not breathe at all and will also trap moisture inside if you wear them. There are nylons just as light, or even heavier with DWR finish that might be better for a wearable garment or something enclosed, like a tent liner.
I love making stuffsacks out of Sil scraps. Buy a bunch of shock cord and cord locks and go to town outfitting all your gear with new sacks. They are light and durable and water proof except for seams and closure. If you wanted you could seal the seams and make waterproof roll-tops too.
Thru-Hiker has a pretty good instructions. A project popular with ultra-lighters is a dual purpose rain poncho/tarp made from silnylon. I've never tried it.