You can walk into an Apple store and purchase one off contract. I did exactly that when I purchased a Verizon iPhone 6, and I have never had an account with Verizon or any MVNOs that use their network. It's true you cannot order one online though.
It used to be you could order the Tmobile version as the unlocked version. That changed this year. Again in usual form (happens every year without fail right when the new phone is announced) I cracked my screen so I'll land up at the Apple store and order today. That Galaxy S7 edge looks slick but I thought about it and I can't do without iMessage. Also iOS 10 is very good to the point I feel iOS 9 was a beta the whole time. Maybe copy and paste will actually work again. Maybe searching exchange email will work again.