Microsoft has a page buried on its website that details a few third party speech component companies. Unfortunately, the page was showing a few companies that are slightly out of date (as of today - although I've requested to the appropriate authorites that the site be updated immediately). At any rate, I'm pasting the information and links that I think SHOULD appear on that site. (Please note, if they are the same - that's good news that they've updated the site).
Here are a few companies that offer third party TTS voices for download:
AT&T Natural Voices
Feel free to comment on others that you know about.