I went for the NTG-2 for that exact reason when I used to shoot with my HV20.
Sure, there are better mics out there, but they also require a lot more in terms of equipment.
With the NTG-2 you can plug directly into the mic input on the HV30.
It's not going to give you the best audio in the world, but I think if you are in a controlled scene you should be able to handle the AGC and other issues.
I have found that audio playing through a pair of monitors does NOT sound exactly how it's recorded on the HV series, so do keep that in mind. That may be because the sound is "pre-boost" or a byproduct of the analog->digital conversion... not 100% sure as that side of things isn't my strong suit.