I got my recent barrels from craigslist - found a local guy who recycles barrels from various sources and had all shapes and sizes of containers. Better yet, he had barrels rated as food-safe. The ones I got contained the syrup for making mountain dew.
On the other hand, as koiguy mentioned, car washes are also a good source. There used to be a truck wash up on the interstate that gave away their empty barrels, and this was the source of the filter for my first pond. Of course this is something you want to rinse out very well. Hose it out with a strong spray several times, then set it up on your pond with a pump to run water through it for a few days, changing the water once or twice. The longer you can flush it out, the safer it will be. Just make certain to do a *complete* water change again a few days before getting new fish... you don't want any lingering chemicals in the water.