Been looking around for some of this stuff to make a tarptent out of, and found this place- http://www.noahlamport.com/store/
The "silicon coated nylon ripstop, zero porosity" for $2.75/yd is exactly the same stuff that is being sold at other places (thru-hiker.com) for $5.45/yd. Does have a minimum 10 yd order, but at that price you could not only do a tarp, but a poncho and ground cloth as well, which I just did, and cut 5 pounds in the process. Also, if they have a precut roll within a few yards of what you order, they will just send the extra fabric, no charge. I ordered 16 yards, got 20, no extra charge.