My 3 sources for cheap plastic barrels.
First is craigslist. Just make sure you buy barrels used for food storage. Very common in areas with vineyards or agriculture.
My local home brewer shop gets all his malt delivered in plastic drums. When he sells out of the malt he puts the drum outside for the taking. I live near by so it's easy for me to check.
Building recyclers. These are junkyard like places that specialize in recycling building materials like used windows and sinks, scrap lumber etc. frequently they have barrels so it might be worth a call.