How do I buy music files if I have an Apple Music subscription

You can purchase music, with or without having an Apple Music subscription, by using the following instructions: Buy music from the iTunes Store - Apple Support

Apple music is a subscription service. If you stop paying, all of the music goes away. Period. It will allow you to download music for offline listening while you have a valid subscription. The other option you have is to use Itunes to purchase music and play it in Apple Music. If you purchase music in Itunes, you can download the file to your device, and it is yours to keep regardless if you have a valid subscription or not.