Find your locale freecycle and ask them.
I asked the same question, and within 3 days I had 12 buckets with lids -- free.
The were all used, empty cat litter buckets (unscented, no odd chemicals for all but 1). They are pretty heavy duty, as they come containing 40lbs of litter.
Since they are square, (and stackable even while full) I have even used one as a temporary tank -- I put a small HOB on it and left it filled with water. No bulging, or anything, and the 6 fish I tossed in it seemed to do just fine. (I was testing to make sure nothing odd was left as a residue)