Electro-Voice - RE50B

This is a different idea from what others have suggested: Use an omnidirectional mic. My favorite is the Electro Voice RE50B. It's the favorite of TV news during hurricanes or Nascar races because you can work it as close as you want without any proximity effects or "popping". That's the key. Since it's omnidirectional it is immune from "proximity effect" that makes voices boomy when too close. It also is almost immune from blasting because of its double wind screen. Finally, it is internally shock mounted which gets rid of almost all handling noise. Use an RE50B (or its less-expensive housing the 635A) and your lips can touch the windscreen and still sound fine. You get rid of outside sound because your voice is by far the loudest thing the mic can hear. Hope this helps!

Phew, depends on whole a lot of factors. For an easy-to-use and general purpose mic, I really think more people should try the Electro Voice RE50 (the 635a model was extremely similar, but is not currently available new). That's for more of an up-close spot mic that won't get overly boomy because of proximity effect and resonances coming off the guitar body. You may also be able to find a used clip-on DPA 4099 instrument mic in your budget. Remember to consider what would be comfortable for you to manage by yourself, if you're working alone. Juggling too much equipment can take your attention away from the instrument. For what I personally like to hear in a recording, my "favorite" would to be blend a spot mic (like the above), with a mid-side stereo pair of condensers. This is definitely more for a professional recording situation, but my favorites right now a Sennheiser MKH8020 / MKH8030 pair.

Are you interested in "one source" in general (e.g. maybe a bird, or a train), or specifically for interviews? You may get somewhat different answers. The choice somewhat depends whether you'll do your interviews in a relatively quiet and acoustically good space, or whether it you'll be working in an uncontrolled and possibly noisy environment. A cardioid mic will \*somewhat\* reduce unwanted sound pickup from other directions, but it is \*not\* at all like a zoom lens or spotlight. It still picks up from (almost) all directions, it's just somewhat stronger directly in the desired direction. In a quiet, acoustically good room, you can use a cardioid from a foot or two distance, and get good results. However, cardioid mics tend to boost the bass as they get closer to a person's mouth, which can be a drawback for interviews. Some people actually prefer omnidirectional for interviews, because you can use them very close to a person's mouth without the audio becoming "muddy" or "boomy" whereas a cardioid mic might do so. When you get close, you can turn down the microphone gain, which results in lower level of background noise. For example, a very popular interview mic, which you'll often see used by TV news reporters, is the Electro Voice RE-50, which is omnidirectional. The Rode "Reporter" is similar, but a bit less expensive. Either one of these should produce recordings good enough for broadcast.