This is the Joyclamp: a clever accessory that lets you attach your Nintendo Joycons to your smartphone or tablet. It lets you play games on your mobile device like a pro. The Joyclamp has a compact design with nylon plastic construction. It is pretty easy to use too: simply attach it to your device, insert your Joycons into its gliders, and pair them to your tablet or smartphone.
The Joyclamp can be attached to the side of any smartphone or tablet (14mm depth). Biodiance is selling this on Tindie.[where to get it]