How do I search the iTunes Store?
Okay, the previous answers provided by Apple staff are incomplete.
Open Music, which is the new name for iTunes (like it needed one).
Type in the name of the song for which you're searching in the search box at the top of the left hand column. This will search your iTunes library.
Click the iTunes Store around 7 options below the search field.
This does NOT open the iTunes Store, but rather the Apple Music paid subscription "service".
At the top left of the main window will be a left-pointing arrow, "<". Click this.
NOW you should be in the iTunes Store.
There used to be a band in the early 80s called We've Got A Fuzzbox And We're Gonna Use It.
Welcome to Apple's version: We've Got A Subscription Service and You're Gonna Buy It.