Hey There 🙋🏽♂️
This feature is originally available in iTunes Store and you're still able to purchase single Songs, Albums, Ringtones and etc.
Once you purchase a piece from iTunes Store it'll also be available on your Music app.
For that matter you'll need to download the iTunes Store App:
https://apps.apple.com/us/app/itunes-store/id915061235
If you're using a Mac simply go to your Music App and on the top of your screen on your Bar click on:
Music -> Preferences -> iTunes Store
The iTunes Store should appear in your List.
If you still need further assist with that matter feel free to contact Apple Support
Hope that helps !
Best,
~Aryan