iTunes Store

In the Music app on a Mac, click in the search box at the top of the left sidebar and then click iTunes store at the top right of the main window. See the screenshot below from Buy music from the iTunes Store - Apple Support

Also see item 3 in the instructions below.

Buy music from the iTunes Store in the Apple Music app on your Mac

1. Open the Apple Music app.
2. In the menu bar, choose Music > Preferences.
3. Go to the General tab, select iTunes Store, then click Ok.
4. 
5. 
   
6. Click the search field, then enter the song or album that you want to buy.
7. Click the iTunes Store button in the upper-right corner.
8. 
9. 
   
10. Click the price next to the song or album to buy it.
11. Sign in with your Apple ID and password to complete the purchase. Need help?

After you buy music, you can find it in your music library.