Many times free 55 gallon drums and other gardening supplies are found in the Free Stuff section on Craigslist.com. Alternately, a local community recycling site like Freecycle.org is a good resource.
Another possible alternative is to call around to factories and businesses in the area to see if they are willing to give away discarded drums. Getting barrels for free with this method can be somewhat difficult, but some companies are willing to sell them at low prices. Just be aware that many barrels that are recycled from industrial use can contain hazardous materials, so make sure to wash them as thoroughly as possible before use.