"Elytra are found in end cities. One pair of elytra is found within an item frame in each end ship."
http://minecraft.gamepedia.com/Elytra
"End cities naturally and randomly generate in the outer islands of the End, which are accessible via the end gateway portal that generates after killing the ender dragon. They are fairly rare; one can travel for thousands of blocks without encountering a city."
http://minecraft.gamepedia.com/End_cities
"End Ship
There is also a small chance for an "end ship" to be generated outside of end cities. These floating structures are built similarly to classical pirate ships, and are made of the same materials as end cities. End ships often generate in front of a pier, which is simply a bridge with a small open arch at the end, from which one can throw an ender pearl to reach the ship.
On the outside, the upper deck is relatively empty, with the only significant objects being a ladder that leads to the top of an empty crow's nest, a single staircase that leads up to the poop deck, and an entrance to the cabin below.
The inside of the ship is rather small. The first room inside the ship contains only a brewing stand, with two potions of healing. A staircase leads below deck to the treasure room.
In the treasure room are two chests that often contain highly valuable loot, and an item frame holding an extremely rare Elytra – all of which are guarded by a single shulker. The floor of the treasure room is lined with obsidian.
A dragon head can also be seen at the bow of the ship, making this currently the only natural way to obtain the beast's head."
