Zedge is a popular app and website for ringtones and notification sounds. A quick search for "Facebook Pop" or "Facebook Notification" often yields the original sound Zedge.
Subreddits like r/ringtones or tech forums often have direct links to high-quality .mp3 or .wav files uploaded by community members. This is often the best place to find the "Original High Quality" version. facebook pop notification sound download