You can buy the Xiaomi YI on a lot of webshops.
I try to list some of them…
Ebay – http://www.ebay.com
Banggood – http://www.banggood.com
AliExpress – http://www.aliexpress.com
Newfrog – http://www.newfrog.com
Tmart – http://www.tmart.com
Light in the box – http://www.lightinthebox.com
GeekBuying – http://www.geekbuying.com