It depends what you mean by "decent". If you mean a HK$20-30K all-mountain/DH rig, no. But if you're not too fussy, then the closest one to a trailhead is probably Friendly in Lantau. You'll still have a 15-20 minute ride to the Chi Ma Wan trailhead, but it's scenic and pleasant.
However, if I were you, I'd just book a ride with Steve of CrosscountryHK and he'll provide you with a bike, guidance, and save you lots of hassle in the process.