I have found that most shirts have labels in them regarding contents. I only purchase 100% cotton and some of them are beautiful plaids. I also purchased some with orange/black/brown/and greens for a jack-o-lantern quilt I hope to make someday. It and a spider web quilt made from plaid strips are the reasons I joined a quilt guild and began this addiction. The men's shirts at my Goodwill are $2.50 and if you look for L, XL, and XXL. You can get considerably more than a yard of fabric, and a good selection of buttons from just one shirt. I usually take them apart while I watch TV.
My main problem is I usually find several shirts I would like to keep to wear around the house or while gardening. But that is becoming to be less of a problem as I dismantle more and more shirts. Happy hunting and enjoy all available resources.