So you have a problem. Because the fabric makers aren't bound by as nearly tight standards as you and I need for using a patched round ball.
So fabric, as I'm sure you know, is often treated before it's put on a cardboard form, and then sold as a bolt of cloth. This makes it easier for the manufacturer to measure out the proper amount, easier for the fabric shop to do the same, AND it reduces the amount of damage to the cloth from the machines that are winding it around the form.
Because of this stuff, often called "sizing", most folks wash their patching material to remove it and to tighten up the weave. This then results in a thicker fabric than when we measured it at the store. Which is often a good thing. The problem comes in when we have found a fabric that when washed, was say .015-.017 thickness, and worked very well in our rifles. BUT the manufacturer changes something..., and when washed the stuff shrinks up more, and it's now .020 thick when compressed. NOW our round ball is too tight with that patch for our bore, and we are back to "square one". That tiny increase in thickness after shrinking won't normally matter to the fabric company, AND could easily revert back to the old results, in a month or a year.
It's very frustrating.
So about the only thing that you can do if you can't get to a fabric store, is to look for fabric sellers online, and see if they have "samples" or "doggie bag" offers, where you can buy a small piece of cotton, linen, or a mix of the two fibers fabric, large enough to wash and shrink and test the fit in your rifle.
LD