Unfortunately, there is no acceptable way to do it at this point. During the first three days or so, there was, but that window has closed.
One user pointed out that some of the big box stores (ie; walmart, Target, Costco, etc) might still have devices with iOS6 on them, so if you feel strongly enough, going that route might be the answer. No Apple store will likely have any older system devices in stock anymore, but it never hurts to inquire. Good luck