I absolutely hate ios 7 and ios 8... If I buy a sealed iPhone 5 from Best Buy (and considering that the iPhone 5 was dropped from the device portfolio in favor of the 5S and 5C last year) is there any way it would be running ios 6?
At this point, ebay or amazon are your best bets. I ran into very good luck on ebay 2 months ago with a reputable seller that was selling new iPhone 5 devices. Just came with device and accessories, no box.
I was very specific with what I wanted: 6.1.4 (the latest for iPhone 5), 64gb capacity, AWS compatible for T-mobile and regional networks (a1428 models released after April 2013) and factory unlocked. White was my preference and I was lucky that is what I got, but I would have taken black if white was not available. They had 1 left. Battery has been amazing.
You may recall, lots of battery issues on early iPhone 5 devices, they fixed this issue on their later iphone 5 models.
As an extra bonus, I was pleasantly surprised when I discovered also that shockingly it had limited warranty until Feburary 2015.
Don't give up, it may take time, but you are bound to run into luck eventually. If i were you, looking for an iOS 6 device for the long term, I would be looking for the latest firmware of iOS 6 (6.1.3 on 4S or 6.1.4 on 5).
IMPORTANT - not all sellers show iOS 6 or the firmware in the title. You have to go through several listings to run into luck and contact many sellers. Keep in mind any auctions with ios 6 or 6.1.4 in the title will get lots of hits and will likely sell for a higher price.
Best of luck!