Your iPhone is already an amazing camera for filmmaking. You are still going to need lights, microphones, and other gear to get the most out of it. The SHIHONG Video Rig for iPhone can help. It fits most smartphones on the market and lets you mount lights, microphones, and other accessories.
This smartphone rig has non-slip handles. It has a bunch of 1/4″ holes to give you some flexibility attaching your other gear. The phone clip has a range of 1.8 to 3.66″. Not too expensive either.[where to get it]