I don't know about used ones but if really urgent need one, you can look for some pharmacy stores, but not all pharmacy stores sell it. Ranging around 150~400+ depending on your need and quality.

If you don't want to use it anymore, I suggests you donate it to HKL. But they only open door for these equipment donation during office hours, and don't come during Friday lunch & praying time, if not you gonna wait until 3pm++