We do not plan on carrying anything like that in the near future, but we appreciate the recommendation. Have you seen our DC Barrel Plug to 2-Pin Terminal Block Adapter? You could probably make your own connector with the standard 9V battery snap connectors with leads, and you could reverse the polarity of the inputs to get a center negative barrel plug. For longer wires, you might just have to splice wires in between the terminal block adapter and the 9V snap connector's leads.
By the way, a 9V battery is not really ideal for robotics (unless you are powering low power devices like microcontrollers). You can read more about why in this blog post.