As I have said, Hong Kong is always know as the shopping and food paradise, famous food in other countries will also be available here.
You can now find it everywhere on the street for about $8-20HKD. Lately, this kind of Taiwanese beverages shops are all over HK. Well, hopefully when you come visit, they are still along as HK is also famous for following the trend so if the trend of pearl milk tea is out, you couldn't find them easily.