This one's airline approved, and intended for pets up to 16 pounds:
http://www.petco.com/Shop/petco_Product_…
I've never had a cat in one, but my puppy was quite comfortable in her Sherpa bag for a long plane ride. I think you might be able to find them cheaper elsewhere, and there are other versions that might be less expensive as well (I think this particular one has all kinds of unnecessary bells and whistles).
Here's something similar that's a little less expensive, and intended specifically for cats (if that makes a difference):
http://www.amazon.com/Jakks-Pacific-1939…
Here are a few more:
http://www.petfoodexpress.com/petfood/de…
http://www.petfoodexpress.com/petfood/de…
http://www.petfoodexpress.com/petfood/de…
http://www.petfoodexpress.com/petfood/de…
http://www.amazon.com/Blitz-70500-Softsi…
You might also want to look through Amazon.com's list of cat carriers:
http://www.amazon.com/b/ref=amb_link_367…
Keep in mind that you're alone allowed to carry one animal per person, and you'll need to verify that the airline you're flying with allows pets, and make a reservation for your cats at least 24 hours before your flight.