Over the summer I read about the medicinal uses of Mullein. That's the low growing plant with the real broad and soft fuzzy leaves and it grows wild in most of the US.
One of the things they mentioned was using the leaves to make a tea, so I gathered some and tried that. It's pretty good, but while I was doing it I thought I'd try stuffing our "Magic Bullet" blender with leaves instead of tearing them up by hand.
It took a bit of fiddling around to get them all chopped up real fine but what it ended up with was a very fuzzy material that is very light and very lofty. I put it in a plastic baggie and compressed it several times and it seemed to recover it's loft easily.
It looks to have the characteristics of down. Of course I'm no expert and I've not done any real testing with it at all but it sure appears to be worth looking into. The stuff literally "grows like a weed" and is easy to harvest.