Pet food containers are a classic option for home brewers storing grain. The other one is 5 gallon plastic utility buckets (I'm assuming there are equivalent sizes in other countries), and you can buy lids for them that make them airtight with a gasket (brand name gammaseal, is believe).
OP if you are looking to reuse something, one option I've used in the past are frosting buckets from grocery store bakeries. The cheap cakes you can buy usually dont use house-made frosting; they purchase big buckets and discard them when empty. You can ask at the counter for a few, and they are usually happy to give them to you rather than throwing them away. They are essentially the same thing as the big utility painter's buckets, but you know they are foodsafe, and they are a bit smaller (maybe 10 liters?).