I've never bought clothing to repurpose but do buy fabric from thrift shops and yard sales. I have bought a few aprons to repurpose too. Here is a link (one of many out there) to "fire test" fabric for fiber content. Since I do buy the fabric at thrift stores this has came in handy for me many times.
http://www.diychatroom.com/content/f...-8e225c257434/