There are several types of stabilisers for gopro but mainly balanced gimbaled (using wieghts) or brushless gimbal (using electronic)
For the first category, Cathay Photo is the official distributor for the new Steadicam Curve. But I think the previous steadicam smoothie gave better results (when well-calibrated.
You can buy cheaper steadicam clones and calibrate yourself. Sgcamerastore sells the Opteka which will balance small cameras - gopro as well.
For a departure from the steadicam design, look into the Stabylcam - a great kickstarter project:
[video=youtube;oKbk7a4PH58]https://www.youtube.com/watch?v=oKbk7a4PH58#t=29[/video]
https://www.kickstarter.com/project...eady-video-with-iphone-gopro-and/posts/877092
For brushless gimbal stabiliser, Orient (Sim Lim Sq) carries the Feiyu tech 2-axis stabiliser. Someone is selling second-hand on Marketplace (video equipment) If you want to buy the latter, bring your gopro to the shop and check that the unit you want to purchase is properly calibrated at the factory and does not exhibit tilted horizon. You have no way to correct the tilt unless you're an electronics engineer and a good hacker! Note that the feiyu does not fit the gopro LCD back so bring your smartphone along as a viewfinder.