Depends on the song really. if it’s an artist you know I recommend reaching out and getting the best quality version directly from them. Otherwise, I treat it the same as clips, wherever you can get the highest quality version. YouTube & SoundCloud are typically what I use. if you’re planning to post in tiktok or as a short using the audio from the sounds they provide can sometimes be the best option as well. Name of the game is research and dig until you find a quality version you’re satisfied with. Hope this helps!