If you are buying it from the Apple Store for full price, any of the AT&T, T-Mobile, or Verizon versions will be unlocked. Just tell them you want to buy the phone "device only, contract free." I bought the AT&T iPhone 5 in 2012 and Verizon iPhone 5s in 2013 from the Apple Store, and the T-Mobile iPhone 6 this year from apple.com. I have used them with AT&T without any issues. I did get a text message this year from AT&T stating that I needed to replace the SIM card to "activate all the features" (it still worked even with the old one), but that was taken care of with a quick trip to an AT&T store. I think it might be that the SIM in the 5 isn't provisioned for VoLTE while the SIM in the 6 is.
I tried looking for the AT&T version and cannot find it anywhere as well.
Going to the apple store feels like a kid in the candy shop and the shopkeeper talks as if we are bunch of kids asking for a small treat.
Can't stand those 16 year old 'so called geniuses. Last year I had walked in to ask for a 5s and the person rudely replies. If we had it in stock we would have a board that says iPhone's in stock. To which I replied, when does apple put in stock signs for Mac's or AppleTV's. to which he said. "Like i said" anyways. offtopic.