As above, Flying Ball is the best ‘high end’ shop, but also take a ferry over to Mui Wo (Silvermine Bay) on Lantau, where you’ll find ‘Friendly Bike Shop’ and a Merida dealer (can’t remember the name).
Bike stuff in HK is a bit limited in choice, and won’t be any cheaper than you can get it back home. Even the traditional ‘bargains’ of cameras, sunglasses and mobile phones aren’t such good value these days…
My advice would be to forget the retail therapy and rent a bike from one of the shops on Lantau. They’ll point you in the direction of a great trail with fantastic views of HK harbour. It’ll be real hot and sweaty this time of year, so get them to ferry you to and from the trailhead in their van, and then take a swim at Silvermine Bay and enjoy an al fresco seafood dinner before taking the ferry back to town… 😀