I started brewing 10 gal. batches, built up a big back log of beer, and ran out of bottles. So I started scavenging on recycling day on my way into work. I have identified several source houses. It's amazing how consistent people are - some people never drink, some others always drink wine, beer in cans, or screw tops. And there are some households that put out a couple of six packs of pry-off bottles every week. (A subset of these include smokers who drop butts in their empties, etc. but you learn to avoid those houses quickly.)
Obviously, you need to give them a good rinse and a soak in PBW. But they clean up just fine. (Soaking in a bucket of warm water with a little amonia takes most labels off easily.)