I discovered fabric in a thrift store when looking for yarn. I also look for zippers, often still in their original package, unique buttons still on their original card and other sewing/crafting materials. I make sundresses for children in distressed areas such as Haiti and Mexico. I use pillow cases in good condition, they are perfect for certain sizes.
I have never used clothing for fabric but have a good friend who does all the time.
In otherwords you are not alone in being thrifty by using thrift stores as a fabric source.
(P.S.) I am new to this and have never ever sent a reply to anything. Hope I did this alright.
(P.P.S.) I have made a small flannel rag quilt and a flannel rag jacket. I found flannel fun and easy to work with, altho a little linty.