Dave at Oware is great to deal with, and has the fabric. I would definately use him for a source.
Unfortunately, making a single stuff sack is less economical because you have to buy minimum quantities of fabric and other materials, and shipping cost won't be reduced much. I usually make my stuff sacks from leftover material purchased for other projects. If you wanted to make several stuff sacks, your cost per sack goes down considerably. With a yard of Spinnaker, you could make as many as 10 small stuff sacks, or a fewer combination of larger and smaller, for about $40.