At the local plumbing supply store. Schedule 40 PVC, available in different widths. You'll need to by buy one whole section which is good enough for two rod cases. Buy two end caps, and two screw on cleanout fittings. Then duct tape an old web belt to it for a shoulder sling. Oh, cut a little foam to put in each end cap.
The airlines all take them, I've flown all over with them. It's at least 10 times stronger than the Plano case (which I also have and don't like). It'll break the plane before it breaks your rods. For security, duct tape the $#%^ out of the end. Luggage thieves at the airport generally look for small stuff to grab and something they can get into in seconds, not minutes.
It isn't pretty but it's hands down the best type of carrier to use. I think it cost me about $18.00 to make two carriers.