I just ordered an airline approved crate for Loki. The one we currently have is wire, so that won't do for airplanes. I found shopping for crates to be really annoying. It's difficult to search for the best price. There are so many different names for the crates with different meanings for "small," "medium," and "large." On top of all this, if you are shopping online, shipping can be quite expensive.
In the end, I got a medium-sized Sky Kennel by PetMate for $47.99 from Dog.com
Because they have free shipping (cough, cough) for orders over $49, I picked up a random cheap dog toy for $1.19.
Here, free shipping doesn't really mean free shipping. It means they drop the $8.95 base rate for shipping, but they still charge you $0.75 per pound! So, with their "free shipping" deal, I still paid $13.50 for shipping.