Search Craigslist, and even a thread in the classifieds section here. If you have a local club, ask them to post a message internally. Homebrewers are often willing to help.
Throw a party (I used to do this for poker parties), and require everyone who wants to drink to bring non-screw top bottles. After hearing about one of these parties, I had a friend (non brewer) who brought me 9 cases of empties. SCORE!
I've done the bar thing for wine bottles. Got thousands of bottles over a few months from a local wine bar.