I think you have been given some very good answers with regard to the identification of pillow ticking. Any clerk in the fabric department can show you the pillow ticking, just ask. Another good fabric that is usually a bit thinner but just as sturdy is "cotton drill" used to make pockets. It is sometimes known as "pocket drill" but not every clerk knows it by that name. Just ask for cotton drill that is used to make pockets. It usually ranges in thickness from .015 to .017.
As someone else told you, be sure to wash the fabric before using it. It has a starch-like substance in it to make it look good on the bolt in the store. One caveat is to either run the cut edges through your sewing machine to keep them from raveling in the washing machine. Or put it into a mesh bag to wash it. If you don't, the ravelings will tangle all around the fabric and anything else in the washing machine. Of course, you could wash it by hand in a pan of soapy water, rinse well and let it dry on something like a clothes line....if you have such. In most places, the one time ubiquitous clothes line has totally disappeared. Once dried, press it to remove wrinkles and make it easier to cut or to tear into strips. I prefer to tear mine into strips, its much easier than cutting and can give strips of a more consistent width. With the strips, you can either choose to cut your patches at the muzzle or pre-cut your patches. If you choose to pre-cut, for ease of cutting, just cut them into squares. Square patches work just as well as round ones and are much easier to cut.
As mooman76 said, look through the remnants pile before buying, you can often find just what you are looking for at a deeply discounted price. Nevermind what color or print is on the fabric, that has absolutely no effect on how well it will work for you. Just be sure that it is 100% cotton. Do not use any synthetics or blends as they will melt in your barrel resulting in a hard to remove bunch of crud in your barrel. "A word to the wise is sufficient." :thumbsup: