Any food service (deli, grocery store, restaurant) place will get similar plastic food buckets in various sizes for things like fruit salad, pickles, frosting, etc. I get 2-3 five gallon pickle buckets every week from a local deli. I've only been doing this for a few months but I already have a large surplus - and this is just one store.
It's really just a matter of making the right connection and you'll have more buckets than you can use.