At the back of Wing On department store in Sheung Wan (near Central) there is a store that sells gym equipment, You might want to try there, though I forgot the name (begins with 'K').
The store near Wing On is called "Kettler." They're a German-owned manufacturer of fitness equipment. You must have a pretty decent sized apartment if you have space for a barbell. I get my fix at the gym
Kettler do exercise machines but there is another shop selling weights, swiss balls etc just close to it that would probably be more what you are looking for. It is on the corner of Connaught and a little street going through to Des Veoux Road.
