I guess it all depends on whether you're exclusively looking for freebies or willing to pay for it. Magix is providing additional audio content (which is particularly suitable for Music Maker) in the form of sound pool collections on DVD or via the Catooh online market platform:

http://www.magix.com/uk/sound-essentials/
http://www.catooh.com/

So, if you want to stick with the structure, consistency and quality that you're used to from your Music Maker sound pools, you might want to look into this. Of course, every new Music Maker release also comes with new audio content.

If you're looking for freebies, you might want to scour the web for sites like these:

http://freesound.iua.upf.edu/
http://ccmixter.org/view/media/samples

Naturally, your booty from this kind of downloading spree will be a lot more random and inconsistent than buying a Magix sound pool, which might or might not be ok for you.

There are also tons and tons of commercial third-party providers of audio content and sampling material, which would work perfectly fine with Music Maker, as long as the content is available in non-proprietary, standardized formats (e.g. audio CD or WAV files). However, high-quality sampling material, including the license to actually use it in a commercial/public context, which is often compiled/created by renowned musicians or producers, can be pretty pricey... I'm probably out of the loop on good, current online shops and providers for this kind of stuff, but 10 to 15 years ago, as a teenager, a lot of my savings went to these fine people here: :-)

http://www.bestservice.de/

Here, some more random links:
http://www.loopmasters.com/
http://www.mi7libraries.com/main/samples/

I hope this helps.