I'm apparently ridiculously bored this afternoon, as I started looking for something to solve this problem and was considering building one from parts.

Start with one of these in the 4-conductor model. Once you have that you'll need a 3-conductor plug and a 2-conductor plug for the stereo headset and the mic, respectively. Solder it all together and you're all set. Some heat-shrink tubing might help make it look pretty.