Occasionally it is available at City Super in Causeway Bay, which also stocks other imported Enfamil formulas. But, there is never a steady supply. I haven't checked in a while as my child is pretty much off formula.
If your baby has a medical/ digestive condition there are other easy to digest formulas available in Hong Kong - perhaps speak with your pediatrician about locally avalable formulas (they are typically manufactured in Europe for the Hong Kong market) which are easier to find in stock.