Q. Where does NYBG have apple tree espaliers?
The garden that you are describing is the Louise Loeb Vegetable Garden at NYBG. It is behind the pavillion in the Home Gardening Center which is across the path from the Hudson Garden Grill.
Sadly the espaliered apples are no longer there but I have some photos from when they were first planted that may give you an idea of their arrangement and the technique used to tie them in to the split rail fence around the vegetable garden.
Both of these photographs are from 2005.
The second photo has a closer look at the ties along the fence.
In addition to the apples, there was a pear tree espaliered along the composting area fence. I don't know why they were removed but they lasted for close to ten years.
I am attaching a link to a nice "how to" article from the Chicago Botanic Garden about espaliering apple trees.
Courtesy of the NYBG Plant Information Service