Buying a SIM-free iPhone 7 does not mean the device cannot be used with a SIM card; it simply means you're free to insert a nano SIM from any carrier and it will work with all of them.
To get an unlocked iPhone 7 (or iPhone 7 Plus), go to Apple's Online Store and choose your desired model. Then, instead of picking a carrier on the left, choose the "SIM-free" version on the right.
The SIM-free versions of the iPhone 7 will ship fast (1-3 business days) in most colors except in Jet Black, which takes 3-5 weeks to ship.
