I do use old clothing, as well purchased fabric. And yes (gasp!) I have even been known to mix them in the same quilt. I don't do it because I need fabric (I definitely do NOT) but because I learned how much of the clothing we donate winds up in landfills when it can't be sold. Some of it is sent overseas, but much of it just gets thrown away after a certain time. I love the idea of repurposing items that are less likely to be purchased because they are seriously out of date, are torn have missing buttons or broken zippers, etc.
I have bought men's shirts, skirts and sheets at thrift shops. There's a lot of yardage in a large man's shirt or a very full skirt. I too am particular about the pieces I buy, although I am not averse to a quality blend as long as there is a lot more cotton than poly. I go by the feel of the fabric as much as anything. If you go look around on Pinterest or Etsy, vintage sheets are very trendy right now for quiltmaking. There are folks selling them as fat quarters, charm packs, etc. Just a side note: sheets that are worn or discolored in the center are usually still in excellent condition around the sides.
Also, men's flannel pajama pants and robes, as well as flannel sheets would make wonderful donation quilts in those cold climates.