There are ambi mics popping up allover the place. The absolute low budget winner is the Twirling 720 Microphone, around 160$. It has a peculiar stereo looking output. 2 channels @96 kHz, but the 4 channel info @ 48 kHz is present, interleaved, so it needs it's own software decoder.
http://yun.twirlingvr.com/index.php/...e/lite-en.html
There's also the Oktava MK4012 classic first order 4 channel mic. 799 €. Limited edition.
http://www.oktava-shop.com/Small-and...012-01-12.html
And no, I don't own one
Yet.
Thinking of building a simple electret one. If I can find the time