In the past few months, we have covered a bunch of awesome game controllers for iPhone and iPad. If you know your electronics, you can always build your own. Adafruit has shared a guide that shows you how to make a pinball controller for your iPad using Teensy.
You are going to need arcade buttons, a USB to Lightening adapter, a board such as the Teensy 3.2, and a few other simple components to get started. The controller gives you tactile feedback as you play the game. More info is available here.