There aren't any 'public' double deckers in new york city. The buses that are double decker are aimed at tourists/people wishing to see the sights. Generally they are of the 'hop on, hop off' variety and different ones take different loops around the five boroughs. They cost anywhere up to 70 to 80 dollars however if your shop around companies you may get it cheaper on the internet. Some airlines like Qantas give you a free 72 hours on one as part of airline ticket deals.