I know some people say that they can get 5-gallon buckets for free, or low cost, from restaurants. That might be a possibility. 5-gallon buckets are useful for so many things. I don't live in town, and I don't know any managers at restaurants, so that was not an option for me.
I am an Inside Track Club (ITC) member at Harbor Freight. Yes, you have to pay $30 per year or $45 for a 2-year membership, but for me it has been worth it. I buy lots of consumable items from Harbor Freight and other products that go on sale. ITC members get special deals every month and sometimes that can be a big savings.
About 4 times a year, Harbor Freight has free 5-gallon bucket weekend sales if a non-member purchases $29 worth of goods, or if a member has any purchase, including items that are $1.00 or less. I always have a short list of consumables that I will purchase at the time and pick up a free bucket.
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Over the past couple of years, I have about 20 free buckets from Harbor Freight. And I have enough lids to store my feed (lids cost extra, but I already had some lids).
Each bucket holds half a 50# bag of feed, about 25# of feed per bucket.
The main reason I transfer all my feed into the buckets is to keep out mice. I keep my stored feed in our attached garage, so I don't want to encourage feeding mice. They got into my feed bags a few years ago, so I had to come up with a better storage option. No mice have ever gotten into my buckets.
The airtight lids are just a bonus for storage because the feed lasts longer in the buckets than in the original feed bags.

Also, I'm at an age where it is so much easier for me to move around a 25# bucket of feed with a handle than trying to wrestle around with a 50# bag of feed with nothing to hold on to.