Interesting, I would have thought it would be easy to find given the amount of Japanese ingredients I see in many supermarkets. Have you tried Olivers, or Great in Pacific Place, or Taste in Festival Walk, or ThreeSixty in the Landmark Building?
You can get it in Hong Kong because we bought some about a year ago - possibly from Oliver's. Maybe do a search for 'Gluten' because there are some other people allergic to gluten on this site and there have been some threads before about where to buy gluten free food; places like HealthGate on Des Veoux Road, Little Giants? in Causeway Bay and a few others. Look in the Directory on this site for Health Food shops for a few more because you need to know the addresses as most of them are not at street level.
I'll definitely go to Oliver's first then try the other stores on HK island, and also check out the relevant threads on this site.
It's really good to know that there are places in HK where gluten-free prodects can be bought. The average HK Chinese person (like my Cantonese-speaking relatives) have very little understanding of this intolerance and what I go through every time I sit down for a meal!