I am flying Jetblue and I just got there luggage requirement by email. So look for a carrier that is no bigger than this:
Your personal item must not exceed 18"l x 15"w x 8"h in order to fit under the seat in front of you
Even though you pay for your pet like $100 ,it still counts as a Personal carry on.
They just changed their requirements because when I booked our flights they gave me different measurements. If you get a carrier ,try looking for one that has a soft fabric top that way if it's 9-10" H you can still adjust it and fit it under your front seat.
I dont know how much you want to pay for a carrier but I got a really nice one at
www.tinkerbellscloset.com . You also get a 20% discount since you are a YT member. Just ask her about it!
Genie