You can usually order "lamp" boxes from anyplace that sells shipping cartons. Regular corrugated cardboard boxes that may be 8" or 10" in diameter and four to six feet long, so just find the size that is closest and cut it down to length.
For repeated use, you can buy heavy "cardboard" tubes in a hardware store, that are used to pour cement footings and pillars. These are 8-12 feet (feet) long in various diameters, very hard "cardboard". Or for repeated use, there's always PVC pipe, with proper caps available. Duct tape or packing tape to seal it is recommended.
I haven't flown with a tank but have shipped them. Simply wrapped in corrugated cardboard, no formal "box" or tube involved. UPS and FedEx are both happy with that, so if you are flying within the US, that's a different shipping option, cuts out the TSA crap about opening the tank as well.
Remember, if you are going outside the US, your US DOT certified tank may not be accepted for fills. And if the tank was oxygen clean (i.e. nitrox) technically, it now should be recleaned because you have no idea if the TSA literally got oily fingerprints inside the neck.