As I noted in my first post, it's not an airline requirement but rather a Dept of Agriculture requirement. We fly both Delta and Alaska and I seem to remember that both have something on their websites about it, otherwise I don't think I would have known to,get one. We've never been asked but I've never wanted to take the risk of being asked and not having one then not being able to board the plane. Here's a good link to check:
U.S. Air Travel Regulations for Pets