Sorry everyone seems to have had such bad experiences at Jo Ann's. The one near me in Tucson is a superstore, and they give good service. I just wish they had better fabric! I use the coupos for books and name brand notions - especially rotary blades - the good ones.
I bought rip-stop nylon at Nancy's Notions several years ago - don't know if she still carries it. The Outdoor Weather Clothing fabric stores have it, too. I bought both kits and fabrics from Frostline Kits years ago - don't know if they're still in business. I actually made down ski jackets, vests and a sleeping bag from their kits. I Googled Rip-Stop nylon fabric, and there's a bunch of on-line places to buy it - different price levels, too.
It's really important to use it for banners that will be outside, because they get whipped around by the wind. Also, carefully finish the seams. Ask me how I know!