Its very sustainable, in that regards. 
Edit: citation added.
 "Some 70 percent of the world's supply comes from China, typically from birds killed for their meat. Most of the rest comes from Europe and Canada, from birds harvested for meat or pâté.", http://en.wikipedia.org/wiki/Down_feather#cite_note-24
Caveat: I have no idea if there's some horrible mechanized way to shear sheep at greater scale. But on the smaller scale the most efficient way is to keep the sheep calm and cooperative. The cheapest way to keep large numbers of sheep also seems to be to let them wander over a large area of cheap land (rocky, steep, uneven, etc. -- unusable for cow grazing or farming is just fine). On the Isle of Skye in Scotland there are simply sheep wandering everywhere, often in the roads, as the sheep farmers don't always bother with much fencing.