Stereo Recording Techniques at the Deutshe Welle Radio Training Center is a goldmine of information on stereo micing techniques.
Uneeda Audio has lots of DIY projects and info, including inline pads, phantom power, PZM mods and much, much, more.
Yahoo! Groups :micbuilders Join now.
The Microphone University At DPA Microphones (formerly B&K) has a lot of good mic and recording info.
Start at Ambisonic.Net if you're thinking about building a Soundfield mic knock-off.
The Official Alan Dower Blumlein website has a lot of neat stuff, but unfortunately they seem to have left all the equations out of the patent applications they have posted here. Darn.
Digi-Key has the Panasonic capsules, Burr-Brown IC's, conductive paint and epoxy, and a good selection of other electronic parts.
All Electronics is a surplus outfit, but their selection stays pretty stable. Good source for capacitors, resistors, transistors, multi-conductor cable, and power transformers.
The Electronics Goldmine is another surplus outfit, but their selection changes frequently. They've got a lot of one dollar specials, so if you see something you want, grab it. Good source for miniature potentiometers.
McMaster-Carr Has brass mesh screening. Search for "wire cloth".