You can buy a 6s directly from Apple.

Those come factory unlocked and are absolutely identical in every way to a 6s you would buy locked from a carrier. And the price is the same too.

Technically speaking there are 2 models of 6s, but the only difference is that one of them adds support for an obscure LTE band that AT&T uses or will use in some markets. If that concerns you then buy the "AT&T model" 6s from Apple. It will still be unlocked and priced the same as the identical phone purchased from AT&T.