how to buy unlock iphone X in USA?

Apple is not yet offering a fully sim-free unlocked iPhone for purchase at this time

While technically all 4 carrier models are unlocked, AT&T, Verizon, and Sprint require you to have an active account or create one with them during purchase.

The T-Mobile version however is unlocked, and does not require an account during purchase, but will only work with GSM carriers, and not with CDMA carriers like Verizon or Sprint.

Additionally the warranty on a U.S. iPhone is only valid in the U.S, and you would have to return the iPhone to the U.S. for service or replacement under warranty.

Not yet. It could be 3 months before you can do that. Warranties are not good in countries other than the country it was purchased.