Help identifying 2013 Honda Accord rear view mirror plastic piece

Unfortunately, like a lot of auto parts, you need to purchase the whole mirror set-up.It is part number 76400-SZA-A02 and the price is $125.63 for an OEM mirror. Or you could look up a local salvage yard and check if they have any cars that match your make and model and pick one up for pennies on the dollar or possibly for free(the part you are looking for, although the mirror would probably fairly inexpensive as well)