You can use the Home Depot buckets with Gamma-seal lids. You can get a bucket with lid for something like $7, I think, and it will hold 25lbs of grain (average). Two buckets with lids should do for a 50lbs bag of grain.
You can also look up Vittle Vaults. They are sold on Amazon, and locally in stores like Petco, PetSmart, etc. They come in various sizes, but they also have gamma-seal lids, and will keep grain very well. They are more expensive, around $35-45 each.