Capture Duo and Capture for iPad are not restricted to just the AudioBox iOne and AudioBox iTwo.
Any MFI (Made for iOS) device will work.
Additionally, any audio interface that works with the Camera Connection Kit may also work. Be sure to check the device manufacturers system requirements for iOS compatibility. If it it is not explicitly stated that the device works with iOS, then you may be subject to running in an unsupported configuration. (i.e. at your own risk)
You can also use the built in Mic on the iPad or the Mic on a set of earbuds attached to the headphone jack.