How to find pros to take on maintenance/improvement of pre-existing sites built with Craft?
As of this posting, there isn't another reliable source for Craft professionals. The CMS is so new compared to other systems that a 'base' of well-known, reliable companies/developers hasn't had the chance to form yet.

You could google and find a few, but they're spread out among developers that most likely wouldn't give you good results. I'd stick with the Straight Up Craft list, since you're at least sure they've seen/read about best practices on the Straight Up Craft site.

You could also post jobs to places like Authentic Jobs, and specifically ask for Craft experience, or Straight Up Craft's Job Board.