I wonder where people buy Xiaomi devices. They are not on Amazon. We run a large website about phones and sell other brands all day via affiliate programs. But never were able to find one via which we can sell Xiaomi phones.
There are some grey market importers for other countries, but even those originally have to buy them online at their shop in a country where Xiaomi sells.
Moreover, most US phones are sold under the cheap-up-front-with-2-year contract model, something Xiaomi doesn’t do as far as I know. Getting phones sold through cell carriers takes a lot of wheeling and dealing. More generally, scaling out to new countries takes lots of work on regulatory compliance, advertising/sales, customer service, etc. which they might not be ready to tackle.
They seem to mostly operate from mainland China but seem to have local warehouses from which they can send items via express shipping.
Personally, I found the following site nice and for my use (6" dual-sim phone) good enough, shortly after I set out on investigations to create my own comparison site. They also have links via alibaba.com and some other affiliate sites, so maybe that can help you to find more leads for sellers of devices.
I bought mine at amazon.de.