I finally defeated the Ender Dragon and need to find the elytra. In order to get the elytra, you have to find and End City, but I'm having trouble finding one.
End cities usually generate in flat areas, in "midlands" or "highlands" biomes, which are found on larger islands. One may find several cities clustered together, or travel for thousands of blocks before finding one.
According to Minecraft wikia:
Unlike other structures, End cities are generated in a noticeable grid. They are located only in chunks numbered 0-8 ± a multiple of 20.
For example one possible "cell" for an End city to be generated in is
z_chunks=80-88which is equivalent to the coordinates