Car Seat for Flight to the US

My husband and I are making our first trip back to the US with our 1-year-old for the holidays. We are very confused by the car seat situation for the plane! We've booked with United and want an FAA approved car seat that we can use while we are in the US. Can anyone suggest an FAA approved car seat that we can buy here in the UK? Any help would be much appreciated! Thank you! 

Why does it have to be FAA approved?  Are you planning on using it while flying?  I didn't even know that was a thing.   

https://saferide4kids.com/blog/faa-car-seats-flying-with-children/ 

Learn something new every day. Thanks Nan!