Are there any online communities for trading unmixed ("stem") tracks from popular commercial music?
What you are looking for are called "stems" or "raw tracks" and are generally only made available by artists who actively want to promote remixes of their works.
Some good sources are:
I'm not familiar with any communities that exist specifically for trading stems ("unmixed tracks"), but most music production forums have a section or etiquette for collaboration. Pick a forum that caters to the kind of music you make, participate in the community, and figure out how the local etiquette works.
There are often a ton of remix projects going on in SoundCloud at any given time, but you will have to do some searching through groups and blogs to find them.
Finally, you should be actively networking and socializing with musicians. Talk to people whose music you like. Some will be open to sharing their music, some won't.
There are a few communities my band is a member of that trade stems with other bands - the purpose being a quid pro quo development of remixes/derived material that we then deliver back to the other bands.
This gets a bit of alternative development, helps us all learn, and possibly attracts new fans.
As an example local to me, the United Noise Remix Theory is a loose collection of artists who all know each other a little, and the commitment was to each remix/create a track from stems from one of the other artists - to be released as a free album.
The key is the quid pro quo - between artists this is common, but an artist is unlikely to give out their work for free unless it is as a promotional item.
I don't know of any engineers, producers, or musicians that would post their unmixed solo tracks anywhere. It's kind of like asking the waiter to bring you only one of the raw ingredients instead of the main course.
Perhaps, you are thinking of some software that filters songs for only certain content, like the guitar or voice?