You can sometimes get food grade buckets from
Walmart,
Are you talking about "free" buckets, or buckets you have to pay for? Where I live, we pay about $5.00 for a 5-gallon bucket and another $2.50 for a lid.
I have not found a source to get free buckets. That would be the best, of course.

My frugal alternative to paying full price for a bucket is to wait for the bucket sales at Harbor Freight and our local Fleet store. Usually HF gives you a "free" bucket with a minimum purchase, but if you are a Harbor Freight Inside Track Club Member, sometimes you get a free bucket just for visiting the store. Our Fleet store has bucket sales where you buy a bucket for 1/2 off, but then you get maybe a 10% or 15% discount for anything you can fit into the bucket.
The trick to taking advantage of those "free" bucket sales is to have a list of items that you will need now, or in the future, and have that list ready to buy some items when those bucket sales come around. If you end up buying $15 of stuff you don't need just to get a "free" bucket, then I would say that is not too frugal.
In the past 6 months, I think I managed to get about seven 5-gallon buckets for "free" just buying things I use all the time. There are many uses for those 5-gallon buckets, but lately I have been using them to hold my chicken feed, scratch, and cracked corn. Each 5-gallon bucket holds just over half a 50# bag of feed. With a good lid on top, the buckets are basically water and mouse proof. Plus, it's easier to handle a 25# 5-gallon bucket than trying to lift or move a 50# bag of feed. Another advantage is that the 5-gallon buckets can easily be stacked one on top of another, thereby saving me lots of room in my garage.
BTW, the buckets I have been getting for "free" on those bucket sales are stamped with food grade safe recycle markings. IIRC, you can use #2, #4, and #5 recycle marking codes for food grade safe storage.