I think Soundpools are all '3rd party', in that sound creators provide the sounds, and Magix is the intermediary that packages/publishes them as soundpools. On Magix's catooh.com site, you can see the link for content creators to sell their media. It's not too different from other loop publishers, who release sets that adhere to a certain theme (hip hop, country, rock, orchestral, etc).
I think other loop makers might commission their own loop libraries, which is what sonic foundry seemed to do; that meant that rights of the loops were owned by sonic foundry/Sony/now Magix, whereas rights for the soundpools are retained by the original authors. Somebody please feel free to correct me if I'm wrong.
The vast majority of Music Maker soundpool content is produced by MAGIX own audio content department, i.e. musicians working full-time jobs at MAGIX. On those occasions when external musicians get involved, the full rights for the content are still with MAGIX, which also puts us in the position to grant our users the rights for public and commercial use of the songs made with the soundpool content. There have been occasions in the past, when MAGIX has licensed content/songs from more well-known artists to bring this content to our customers as e.g. remix kits. For that content, a commercial license would never be available htrough MAGIX, as the rights are indeed still with the original artist. Only non-commercial/private use would be possible. However, those examples are far and few between, and you'd always know you're dealing with such an artist soundpool, since the artist name would be prominently mentioned.
Going back to the original question in this thread: I'm not aware of any third-party provider offering soundpools that are specifically formatted as MAGIX soundpools (and thus completely compliant with the soundpool manager in Music Maker). However, you'd be able to use any third-party content from the wide variety of companies that specialize in sample content, as long as the material comes in standard audio file types that are supported by Music Maker (e.g. WAV, OGG Vorbis, MP3). It just won't be possible to manage that content as easily via the soundpool manager ("Loop" section in the current Music Maker generation). You'd need to use the built-in file manager, or drag & drop the samples from your Windows file explorer instead.