Mary Ann, go to the bakery department at Shoprite and ask if they have any large buckets with lids that they could spare. They almost always have buckets waiting to be recycled. The largest buckets I've gotten from Shoprite have been 4.25 gallons. With some aggressive jiggling to get the flour to settle, you can fit 50 lb. into two 4.25 gallon buckets.
When you ask for the bucket, tell them they don't have to wash it, as you'll be washing it anyway. If they have only one bucket, go back another day and ask again. You could also ask at the deli, but the materials they get in buckets, such as pickles, might leave an odor in the bucket.
I have an old house with a basement with moisture issues, and, even though I run a dehumidifier most of the summer, I have a very humid basement. My most recent batch of flour is coming up on being a year old. I wouldn't necessarily recommend pushing it that far, but it is a good barometer for judging how well the buckets work.