I doubt that you'll be able to find a new iPad Mini with iOS 6.
That device would have to be more than eleven months old and I don't think that you can just walk into the next store and pick one from the bottom of a stack of boxes.
Remember that Apple is the company that's kinda famous for having a high inventory turnover rate.
You'll have better luck with trying to find one on eBay or Craigslist, but even there it will probably take a while.
Those who never updated their iPad Mini either don't want to use iOS 7 themselves, so they'll keep their iPad or they don't know about updates, which means they either won't sell their device (even if it were slow, those people don't care) or they won't know that they should write that the iPad is still running iOS 6 on eBay.