Like several have mentioned retail stores, Walmart, K-Mart, Target.
Better than those go to your local fabric store (Yellow Pages). Take with you a small Micrometer 0-1" to check the cotton fabrics, ("Sail Cloth" and "Canvas" can be found in .014 to .019 thickness and everything in between). "Pillow Ticking" usually runs in .014 and .018 thickness. Buy just a 1/2 yard, take it home and wash it when your clothes are washed, this gets the "sizing" out so you can get an actuate measurement. :thumbsup: