How do I buy music from the iTunes Store?

How can I choose and purchase music from the music store? Shouldn't it be easy if they want sales? When I choose a version of Liszt's Les Préludes it disappears when I choose iTunes Store, and there seems to be no mechanism to browse to find an acceptable version. I need to study the music as I will be playing it. Before, I've been able to go to the store, choose the music I want, and buy it. Why so difficult?