Here is a handheld camera stabilizer that allows you to take smooth video with your iPhone or other smartphones. The Zhiyun Z1-Smooth 2 is a 3-axis gimbal with app and wireless remote control. It supports smartphones below 7 inches. Only one battery is needed to use the Smooth II.
The Smooth II uses advanced motors to stabilize your videos. It connects to your smartphone via Bluetooth. You can use this for horizontal or vertical videos. Costs under $350 at this time.[where to get it]