Best Buy (and all re-seller iPhones from places such as Target, Walmart, etc) are the same. But keep in mind they are bound by a US Reseller Flex Policy. This means once a sim card is inserted, they are locked to that network. This also requires the devices to be activated using a domestic SIM prior to use.
This piece of information is different because it's not a "truly unlocked" device due to this policy. Although the internals are all the same, this policy essentially prevents it from being truly unlocked.
Once you insert the SIM and it locks to whichever network that SIM belongs to, you then have to work with that carrier to get it unlocked. This can be a headache depending on the carrier and they may not always perform the unlock due to the purchase location. For example, I've seen horror stories of T-Mobile refusing to unlock the device because it wasn't sold by them and their records don't show a purchase or lock history. Your mileage may vary.
But to answer your initial question, from a hardware standpoint, the phones are all the same. Best Buy specifically doesn't have carrier specific models. They have a single model that will work across all carriers that access all SIM cards from any of the 4 major carriers or domestic prepaid carriers. Whoever told you they only have an "AT&T" model is lying.
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