Even though your question is about "music", you are asking about Movie Edit Pro, which is VIDEO editing software. You should be posting this in the VIDEO section.
I presume that you are using Movie Edit Pro (MEP), but you didn't say and you didn't give the version.
In MEP there is a button at the upper right of the screen, just below export - CATOOH. Click on it. This is the Magix Catooh site and it contains all kinds of things audio, video and photo files for purchase, including some freebies.
However, there are other sources. Rather than using Slideshow Maker for the music, if you don't need to match the slides with the music exactly or you want to do it yourself, then try Soundtrack Maker (in the menu, Edit, Wizards, Soundtrack Maker). Or go and look at what you already have in the 2 following folders:
C:\ProgramData\MAGIX\Movie Edit Pro 2014 Plus\Soundtrack Maker Styles
C:\ProgramData\MAGIX\Movie Edit Pro 2014 Plus\Slideshow music
Go into the folders and you will see many ogg files. These are all sound and music files. You can drag and drop whatever you want on the timeline and create your own mix.
You can purchase SmartSound SonicFire Pro and create music with it. It is better than Magix in that it will do an intro, body and outro for whatever length that you specify (as long as it is more than a few seconds). Their site has a lot of music for videos that you can purchase and use. Unfortunately there is no plug-in for Magix.
There are also other internet sites with free and royalty free music for purchase.