Get NewerTech NuGuard KX Case for iPhone 7 or iPhone 7 Plus at MacSales.com.
Update 13 October: SIM-free any-carrier iPhone 7 models can now be purchased.
MPG called Apple to try to buy an unlocked any-carrier phone (with both GSM and CDMA support) and was told that as of Sept 12, Apple does not offer an unlocked iPhone supporting both GSM and CDMA that is not tied to a carrier, but that it may be coming in a few weeks.
At present (12 September), Apple will only sell iPhones tied to a carrier (even unlocked ones!); see Apple iPhone 7: can an Unlocked Model Actually be Bought at Apple Store?. When I called, my attempt to purchase an unlocked Verizon model was refused because I am not an existing Verizon customer. Apple simply will not sell unlocked phones not tied to a carrier—not yet, but perhaps in a few weeks.
MPG doesn’t like or agree with this anti-customer approach, but there it is.
See Apple iPhone 7 Caution: GSM-only model lacks CDMA network support, precluding future use with Verizon or Sprint for why one might want to buy a iPhone 7 for Verizon when intending to use it with AT&T (for now).