The reason you can't find the music files, is because sdcard/data/ is a system file that does not allow the user to view the contents.
I have taken the following out of a article to help you get the songs you have paid for
(You have to have a rooted phone for Method One. If you do not, then you have to go to method two.
Method One
Getting All Your Music Offline
.....If you just want to play your music offline using the Play Music app for Android, you can stop here. This section will cover how to retrieve the actual mp3 files so that you can take them and do whatever you want with them.
Why do that? Because it’s your music that you uploaded, and for some reason, Google only allows each song to be downloaded to your computer twice. What happens if you pass that limit? You can no longer download any of your music. But downloading the files to your Android device, which doesn’t have a limit, and then extracting the files, gets around that little restriction.
Keep in mind, you’ll need to be rooted to do this, but rooting is simple... Once you’ve got a rooted device, you can get started.
Step one is to download a file manager with root permissions; my personal favorite is Root Browser. It is full-featured, but supported by ads that run along the bottom of the app. For our purposes, it will work perfectly.
Open it up, scroll down, and select “data”.
From there, scroll down and select “com.google.android.music”, then select “files”.Your final selection will be “music”; under here, you’ll find all of your downloaded mp3s. They will be numbered and won’t have names, making it hard to know which song is which, but at least they’re there. Copy and paste them using Root Browser to put your music anywhere else on your phone or on an external microSD card.
Method Two
Open Google Play Music in your computer's web browser. After login, make a playlist with all the songs you want to take offline. Then click on the three dots, and click on download. This is the easier way.