What exactly are you looking for? iPhone or software? The iPhone 5 came out with iOS 6 installed, and it’s latest software version is 10.3.3. I don’t think you’ll find an iPhone 5 with 6.1.2 installed. Getting iOS 6.1.2 for the iPhone 5 will be impossible, because it’s not signed for this device by Apple any more. If you want to buy an iPhone 5 with iOS 6.1.2 installed, you’ll have to search various web shops. Maybe you’ll find it exactly the way you want it. Good luck!
Your best bet would be a factory sealed new old stock iPhone from the first production run. That would be more expensive than a regular used and updated iPhone 5.