I used an app called Multitrack DAW by Harmonic Dog. http://www.harmonicdog.com
I have never tried it with garage band. I seem to remember reading a year ago that garage band would only allow you to record 2 channels simultaneously instead of all 4. This might not be a problem if you don't have plans to record 4 tracks at a time. It has been a while, I seem to remember harmonicdog will take 4 at a time.
Another interface option is the alesis IODock docking station for generation 1, 2, and 3, but not the latest gen or mini with the different connector. No camera adaptor (for USB) is needed. The whole iPad plugs into it. I have not used this item, but I have read that people are using it with garage band. It is two channel. Sweetwater and b&h photo have them for $150. B&h has a used one for $110.