I do the same thing. Near the end of the month the store in my small town, has a rack with a certain colored tag on which the clothes are all 25 cents and then they go down to 10 cents by the end of the month. So, I check that rack for 100% cotton items. Then I take them apart & remove the buttons & zippers and fold the fabric up compactly and add it to my stach.
I have a question. Lots of the blouses now days have 10% & sometimes a bit more nylon or spandex mixed with the cotton. Does any one know if that kind of fabric works well for quilting. I have purchased some blouses with the nylon or spandex, which stretches a bit one direction, for fabric, but not tried it for quilting yet.