Hi Joyce, there are probably good sized buckets available at a big box store, but especially at a farm supply store if you have those nearby.
Here is a post I wrote on my dad's super smart barrel planters he created.
Find a professional painting company, in your area. They usually buy paint that comes in 5 gallon, plastic buckets. Ask if they would save, however many buckets you need. Tell them you will be happy to pick them up. Normally, the paint they use is a water base paint and the buckets can be easily cleaned with soap and water. If there is dried on paint, inside the buckets, soaking them overnight should loosen the paint. Happy planting!
amazon has them online and Target lists a self watering large container for tomatoes
Where do you live Joyce? What stores are available in your area?
Any hardware, home improvement center or nursery will have what you need.
Another option is Habitat Re Stores.
Second hand and thrift shops too.
Ross, Marshalls, TJ Maxx, Home Goods.
Yard sales or garage sales.
Craig's List, freecycle.org, Nextdoor.com.
Or just order on line - many sites to choose from and there is always Amazon!
Talk to your friends with cats. If they're using Tidy Cat Litter products, some of these come in fairly large plastic buckets that would be great for mass veg plantings. This would be a regular supply.....
Or post on Craig's wanted or freecycle.org in your area asking for them.