Where to buy iPhone XS Max?

Apple no longer sells iPhone XS Max. Though, depending on your country, there may be a certified refurbished one. You can check the availability here: Certified Refurbished Products - Apple. Make sure to select your country on the webpage.

Furthermore, you can check if any third-party resellers are selling them.

That model was discontinued 3 1/2 years ago.

You cannot buy it from Apple, you can try to find one on BestBuy marketplace or any place that sells refurbished iPhones .