Garage Band for iOS has a mode to use real instruments. The documentation is unclear, however, how to connect those instruments to the iOS device. What options exist to connect an electric bass guitar to an iOS device to use with Garage Band?
Hope this will help? http://www.musicradar.com/news/guitars/6-easy-ways-to-connect-your-guitar-to-your-iphone-ipad-or-ipod-touch-516544 Or even more specific: https://gigaom.com/2013/02/09/four-ways-to-connect-your-instrument-to-garageband/ Or you meant something different?
I have been using iRig from IK Multimedia. It works great, and is a quite cheap solution.