Premium Responsive Drupal 6 themes?
Update 1: As pointed out by the OP, Jeff Burnz doesn't only sell his time as a consultant, but actually has a site, AdaptiveThemes.com, where he sells his "adaptive"-themes, but those are D7 only.

Unfortunately I'm not aware of any company advertising such themes yet.
One of the first I would except to see this from is http://fusiondrupalthemes.com/, but nothing there yet.

If you are looking for something more unique, I believe the Drupal 8 Design Initiative owner, Jeff Burnz, sells his time to make themes, and is the author of a number of responsive themes already. Sky, Corolla and the base theme Adaptive theme, which you may want to look into, given the lack of an actual answer to your question.