Hi there, and welcome to car-seat.org.
While we do have a
Swap Forum where members sell a variety of items -- and where any reference to buying/selling/renting/etc needs to remain -- it's possible we can help you out without going that route as your timeline is short.
If you're after a CARES harness then I would presume you have purchased a seat on the plane for your child. What are your plans for a car seat at your destination? Is there a reason you can't bring your car seat onto the plane with you, install it and harness your child in it, and avoid the need for a CARES harness altogether? Kids tend to be much more comfortable in their usual car seat, and then you can verify the seat's whereabouts and integrity at all times. However, you might have a seat awaiting you at your destination.
If I've made some inaccurate assumptions here please forgive me, but do fill us in as to your specific travel scenario and hopefully we can give you some ideas. Buying a new CARES harness is also an option.