I've heard stories of Hollywood Video or Blockbuster stores throwing the extras into their dumpsters... You could potentially get a whole bunch for free if you weren't averse to dumpster diving.
Some areas have laws against dumpster diving. My advice: bring a few large, empty cardboard boxes along and place them outside the dumpster. If a cop arrives, point to your boxes and tell him that you're moving and looking for boxes to use. At worst, he'll tell you to stop, but most likely he'll leave and let you keep looking "for boxes."