Sportsmans Guide has a limited number in stock - BOGO free. If you have a membership and a coupon code you can save even more.
Used U.S. .50 Cal. Ammo Can - 805552, Ammo Cans at Sportsman's Guide
These are still ~7 bucks each if you buy an even number.
Probably left over from their S&B .308 sale packed in .50 cal cans. I missed out on that one but the S&B is corrosive so maybe not a bad thing.