我想找一個水氧感應器(sensor)?

球興科學儀器有限公司

It is kind of industrial grade sensor and I doubt if you could buy it in Hong Kong. I know there are a handful number of vendors in HK specialized on that kind of products. I can rest assure you that they won't keep any stock of that kind of sensor in HK. They will show you some catalogues and order one for you from overseas. I doubt if they wil entertain you as a one-off customer.

May be you can just order one yourself online:

http://sensors-transducers.globalspec.com/LearnMore/Sensors_Transducers_Detectors/Analytical_Sensors/Dissolved_Oxygen_Sensors