As an update:
Thanks to JGL for the recommendation for the RAB Aeon shirt and HK Katherine for the tip of purchasing RAB from Protrek.
After checking out the variety of Hiking stores in Mong Kok with no success I headed for Protrek in Yau Ma Tei. I stumbled upon a store called Re:echo ( (Yau Ma Tei MTR Station Exit A2, beside Circle K convenience store upstairs) following Google Map for Protrek and found a Long Sleeve RAB Aeon shirt for $350 HKD.
Re:echo were nice enough to tell me how to get to Protrek but I ended up coming back to purchase the RAB Aeon shirt as Protrek did not have any.