Try the Kettler store on Connaught Rd, next to Wing On in Sheung Wan. Or see if you can buy one through a sports physio. They are expensive.
What do you want one for- static balance or dynamic?
If you want something cheap, try the little sports stores dotted around that sell badminton racquets, etc. Some of them will carry budget versions (about a foot in diameter, no stable platform, only good for balance and not bouncing on).
Or you can get a balance platform (wooden disk with a hard hemisphere on one side) made up from the little woodworking shops dotted over HK. You will need to draw a picture for them though.
Thanks for all your help. I found one at Optimum Performance Studio. A bit too pricey for me though (1200 HKD) but it was the same price everywhere else in HK. They had the full range of the latest gym equipment though.