There's a few choices you have:
[ame="http://www.amazon.com/Zalman-Microphone-Zm-Mic1-Sensitivity-Headphone/dp/B00029MTMQ"]Zalman Zm-Mic1 Clip-on Microphone[/ame]
- Lots of people use this mic for headphone/mic setups. It works as advertised.
(Con: It is omnidirectional, meaning it picks up outside/unwanted noise)
[ame="http://www.amazon.com/Cyber-Acoustics-Desktop-Uni-Directional-Microphone/dp/accessories/B0000511XL"]Cyber Acoustics Mic-48 Desktop Microphone[/ame]
- It advertises unidirectional audio, but that doesn't mean it is or works.
You can also do a few searches on Google (or your favorite search engine) for
Desktop Microphones. Nearly all of them work, it's your decision when it comes
to the final choice; if you find one you like, you can always ask about it here.
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This next one I haven't gotten the chance to try yet...
Stryker PC Throat Mic
There's no reviews for it, but I know the Military has been using throat mics.
Commercial throat mics (such as that link above) are used widely in paintball,
some motorcyclists use them too.
Throat mics cancel out any background noise because they only "read" the
vibrations of your throat, which means you can even whisper and you're heard
(even though there's a loud background noise).
... But like I said, I haven't been able to try it out yet or find any reviews for it.
Once I get the money to, I'd gladly buy one to try it out and make a review for.