In my visits there is very little in the way of Duty Free shopping in New York airports. I suspect you may be better off buying them on your way out of the UK where there is more choice and better prices.. I avoid the shops after baggage reclaim in the UK... they arent actually very good deals so dont get them on the way home.
America does not do duty free shops like we do. There may be one small store and a book/magazine shop and that is it. 200 Marlboro Lites in Boston duty free yesterday cost $27 is that helps.