Should be able to get them in any Mothercare stores. If they don't have them in stock in your nearest branch, just ask them to deliver the colour that you want to the store. I saw them in the Shatin branch.
Also the Learning Centre on the fifth floor in Peddar Street, building next door to Shang Hai Tang has trunkies, in blue and hot pink, but Mothercare has the new 'cow' ones. Very cute, let me know how you like it, I've been thinking about buying one for my little one when she gets old enough.