Where can I buy iPhone XS in USA?

You can find authorized Apple dealers by clicking on the link:

Find a Store

But, keep in mind, that the built in hardware might be different from the device sold in your country. An iPhone is supposed to be used in that country where it has been sold.

Also, the service options might be restricted to that country of purchase as well.

It is different for every shop and seller. I suggest you use Google and visit the online shops of Amazon, Best Buy, Target, Walmart etc...

Here is Amazon:

Space Gray: https://www.amazon.com/dp/B07TJBCC7H

Silver: https://www.amazon.com/Apple-iPhone-XS-Fully-Unlocked/dp/B07K4VCLLK

Gold: https://www.amazon.com/dp/B07K4V35TJ

There might be other shops, that still have older models in stock, for example Mobile Service Providers. But if they have, make sure, to buy an unlocked iPhone.

You need to buy it from Third-Party, i would just buy it online and buy from Amazon for example. Is there a reason you want the Xs and not the 11 Pro, if they are the same price?

Third-party sellers may offer discounts, but it's not guaranteed. It doesn't have to be cheaper by 300$, it can also be cheaper by 100$ or something else.