Is iPhone X availble in the USA
Yes, the iPhone X is currently available to purchase online and in Apple stores.  

However, there's no guarantee that the store you visit will have the X in stock when you arrive.

You can try to order one thru the Apple US store site, select a pickup store, and then check the store stock (all thru the store site).  That's your best way of ensuring a store has stock when you place your order.