I've gotten a few free 55 Gallon Plastic Barrels by looking on Craigslist.
A business was giving away free 55 Gallon plastic Barrels so sometimes you can find them free through Craigslist from individuals or businesses.
If you can't find any free ones then usually you can find some cheap plastic 55 Gallon Barrels for $10.00 which is pretty cheap too.