If you find any ... let me know!
Seriously, as Josh Smith points out in this post, it's amazing there isn't a CodePlex community or something for this. Heck, it is amazing that there aren't more for purchase!
The only one that I have found (for sale) is reuxables. A little pricey, if you ask me, but you do get 9 themes/61 variations.
UPDATE 1:
After I posted my answer, I thought, heck, I should go see if any CodePlex project exists for this already. I didn't find any specific project just for themes, but I did discover the WPF Contrib project ... which does have 1 theme that they never released.
UPDATE 2:
Rudi Grobler (above) just created CodePlex community for this ... starting with converted themes he mentions above. See his blog post for more info. Way to go Rudi!
UPDATE 3:
As another answer below has mentioned, since this question and my answer were written, the WPF Toolkit has incorporated some free themes, in particular, the themes from the Silverlight Toolkit. Rudi's project goes a little further and adds several more ... but depending on your situation, the WPF Toolkit might be all you need (and you might be installing it already).