There seem to be many ambisonic microphones popping up around the place - both high-end and mid-range commercial microphones, as well as home-built DIY microphones.
Here are some photos and links.
(all images’ copyright owned by their respective manufacturers).
The premier ambisonic microphone range by Soundfield is soon to include a (presumably cheaper) A-Format microphone, the SPS200 (pdf link).
AGM Digital are redeveloping their MR1, originally developed with Danish Pro Audio using matched B&K 4011 capsules.
Core Sound are about to release their TetraMic a-format ambisonic microphone.
On the DIY side:
In 2005, I made my own DIY ambisonic microphone using Panasonic electret capsules, which turned out to be great quality for a cost of around $300.
Also in 2005, Etienne Deleflie built his Sound Thief ambisonic microphone using Rode NT5 microphone parts (capsule and phantom preamps).
Earlier, Henry J. Walmsley had already built his own ambisonic microphone also using Panasonic capsules.
In late 2006, I heard of two more new DIY ambisonic mics:
Rafael Kassier’s mic built from Oktava capsules:
and Paul Doornbusch’s beautifully constructed DIY mic built with Transound capsules:
Probably you could find out about more diy ambi mics by joining the Sursound list.