How to buy a song not on iTunes?
I have seen in the forums questions like this, but it has never been answer the step by step on how to do it. Please don't just send a link.
iMac 21.5″, macOS 10.13
Find an alternative source, such as Amazon Digital Music, Bandcamp, Beatport, EClassical, etc., that has the song, and follow their procedure to purchase it. Once you have downloaded it, add it to your iTunes/Music library.
Some songs are simply unavailable for digital download, anywhere. In that case, you can consider acquiring a used CD and importing (ripping) it.