Have you tried ParkNShop? Its listed out of stock online but it maybe available on some shelves. You could also call Mead n Johnson to check that it hasn't been discontinued in HK.
Thanks Rani. We got a sample tin from a pediatrician's office but it seems really hard to buy! Am getting someone to call Mead & Johnson to see if its actually available for sale in HK (the HK website you gave me doesn't include Enfamil AR although the US website does)