I want to know where I can find Du'a recited after completing the recitation of al-Qur'an.

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

As far as we know, there is no particular Du'a for ending when the reciter completed the whole Qur'an that is recorded from the Prophet (Sallallahu Alaihi wa Sallam), that he used it or taught others to make special Du'a for that occasion.
But, it is recommendable for the reader to make Du'a after he/she finished reciting the Qur'an asking Allah Almighty for whatever he/she wants from the good of this Life and Here-after, and His favors.
It is reported by Ahmed and al-Tabari in al-Kabeer from 'Imran Ibn Hussein that the Messenger of Allah (Sallallahu Alaihi wa Sallam) said: "(Read) recite Qur'an and ask Allah with it, before the kinsfolk come (in the future, nowadays) who will recite Qur'an and ask the people with it (money or other things)" .
Also it is narrated by Anas Ibn Malik , may Allah be pleased with him, that he used to collect his family members when he completed the whole Qur'an and make Du'a together with them.
Allah knows best.