where to buy

If you don't live in the US, only Apple sells unlocked iPhones. Buying one from AT&T would mean you'd have use AT&T as your cellular provider and that's likely not a good solution unless you live in the US. Don't be one of the many who come here daily upset because they purchased a phone which cannot be activated in their country.

Also remember, any iPhone you buy in the US only has warranty in the US. iPhone does NOT come with International Warranty. If you need service or warranty support, it will have to be physically brought back to the US.

I can find nothing which suggests Apple sells iPhone at Airports in the US.

Doubt you will find one in an airport.