Like you, I tried Bull Bike but none of the stuff there really fit/appealed to me. I'd check out Gravity Reaction Cycles near Prince Edward MTR (less than 5 minutes' walk from Exit C2). Although they carry a good range of non-DH/FR stuff, they also have a good range of full-facers from Urge, all of which fit me well, although YMMV, along with the SixSixOne all-mountain helmets (Recon, I think). I ended up going for the Urge Archi-Enduro.
You can also check with BMX Evolution, although I didn't find any there that fit my head. On HK side, I know Bicycle World (in Wan Chai) has a few brand, but again am not sure about sizing and availability. I haven't tried Flying Ball.