Here’s two small devices that can be used to turn your iPhone into a portable audio recorder. The first Tascam IM2X is an X/Y Stereo Microphone adapter that offers stereo recording directly into the iPhone and works with a free downloadable app. Currently this item is on sale for 50% OFF now just $39.99 (found here).
Tascam IM2X X/Y Stereo Mic for iOS iPhone
The second option is the Tascam IXJ2 which is an adapter that allows you to directly plug in your own choice of microphones. The iXJ2 records 2 channels of audio at 16-bit/44.1kHz (CD quality) resolution from one of its two unbalanced 1/8″ (3.5mm) stereo mic/line inputs. Both inputs can also be used simultaneously in mono, and even summed to one mono signal from there. The free Linear PCM Recorder App, available on the iTunes App Store, provides optimized functionality. This will be a better choice for anyone who wants to use a LAV microphone (find it here).
Tascam IXJ2 iPhone Mic Input with 1/8″ (3.5mm)