You can buy the charger for Mi band HRX from Xiaomi online Mi Store. Since it is the official store of Xiaomi you can be assured that it is the official charger. This charger can also be used to charge Mi band 2 as well since both Mi band 2 and Mi band HRX comes with the same design. This product costs Rs.129 and comes with 6 months warranty. Just search in Google 'Mi band HRX charger' and the mi store link appears first in Google search.
But if the product is Out of Stock in the Mi store, then I would recommend checking out Flipkart or Amazon and buy a duplicate charger because I don't think the functionality would change. Just look at the reviews to make sure that the charger worked for others.