What's quickest way to get unlock code for At&t N6 topic

I am looking to purchase an At&t branded N6 for use on Vzw...I've done the research and I know for sure that it will work. I also know that I need to get an unlock code for it to be able to work. What is the quickest way to receive a code without having to pay a fee? Thank you so much for your help.

P.S. The seller inserted a Vzw nano Sim to see what would happen, and it immediately asks for the code...does that mean it's already unlocked n all I need is the code? Or does unlocking and activation all happen in one step once the code is entered?