The best alternatives to Flex Cam Oppy are Rigiet, ATOM, and Smoovie. If these 3 options don't work for you, we've listed a few more alternatives below.
The ATOM is the smallest 3-axis smartphone gimbal with a foldable structure, which provides great portability, rich fantastic functionality and expanded App extensions. UPDATE: Atom 2 is here: https://www.producthunt.com/posts/atom-2-2