Go to your nearest doughnut shop or supermarket bakery and ask what they do with their empty icing or jelly/filling buckets. Many rinse them out and toss them into the recycling bin, or leave them near the back door until they have a stack to take to the dumpsters. Many will be glad for you to take them, especially once you tell them what you want to do with them. You might have to come back on a certain day of the week, but some may just give you a stack immediately. Be ready to handle some messy ones by bringing some garbage bags, just in case.
If you need larger numbers of buckets, and live near an industrial area, look for food production companies. They usually have stacks of food grade buckets they are happy to give away, and even may have food grade plastic 55 gallon drums if you want to eventually expand to larger containers.

Good luck!

Io prendo i miei dal panificio del mio negozio di alimentari locale. Ho chiesto gentilmente e sono stati felici di darli invece di buttarli via.