You can buy from macy's, ebay ( some sellers offer international shippment)or usa代購, I bought one britax and one maxi cosi for my daughters ( with iso fix, for 15~70lbs, both front n rear facing), each of them costs hk$2,500~2700 (including freight charge).
there are britax and Maxi Cosi carseat sold at mothercare... and you can choose the right one for your daughter
FYI - i have bought the following for my son, 3 years old now... :) he was 2 and a half when i bought it..
-Suitable for children from 9kg (20lbs) to a maximum weight of 36kg/79lbs) – approx 9 months to 11 years old