Buy beer that comes in bottles you want...always a bargain compared to buying empties retail.
Yard sales and "antique" shops...there's always a swing top in there somewhere. Prices at these places are always negotiable.
Craig's list.
Good old fashioned social networking, not the digital kind. Once you start giving home brew to friends and neighbors be sure that you tell them you are bottle hungry. There's almost always somebody that has been hoarding good bottles but didn't know anybody to give them too.
Like mentioned above. I got quite a few from craigslist and then just having anyone I know save their bottles. Stick them in the dishwasher and then sanitize them and you are ready to rock.