I get all the old swatch and sample books from a local curtain shop, they carry some really gorgeous fabrics, and some of the samples are quite big, like the size of a small curtain... They're only too happy to give them to me - they'd be going in the rubbish otherwise.
Freecycle is a great source.
I've also got people hoarding recyclables like magazines, aluminium cans and glass jars and bottles for me
I'm into recycle crafts mostly, so actually clearing out my own house yielded tons of stuff like old clothes, cardboard, junk mail flyers etc.