We have a deli here that I noticed would have 1or 2 5 gallon buckets with lids sitting outside their side entrance. I asked if I could buy them and they said "help yourself. We put them out and if they sit there for a while we trash them." I got 2 to 3 every week when I was making crappie brush piles. I have since learned that several restaurants will either give them to you or sell cheaply.
Firehouse Subs sells them and donates the money to local firemen funds.
6 gallon is kind of an odd size, but check with some restaurants.