You do not need a stereo mic, you need a good mic.

In the past I have suggested the Audio Technica At897, which is within your price range.

Audio-Technica - Microphones, headphones, wireless microphone systems, noise-cancelling headphones & more : AT897 Line + Gradient Condenser Microphone

Have a look at some of the similar questions asked in this section and you'll see that there's lots of advice.