hey there,
The diagrams/circuit may seem simple but those little panasonic mics are a wonder.... I've used them for recording acoustic guitar, piano, cello and they have such a flat response its silly for the price really... google 'linkwitz mod' for how to improve its handling of higher SPL (volume).
When you become a little more electronics savy you might want to add a small preamp inside the mic enclosure, as close to the mic element as possible, better SNR for longer calbes.
Digikey, Farnell, Mouser, supply the panasonic mic elements...although I think the WM-66A is discontinued. Plenty of alternative ones though, both omni, and uni directional.
Good luck, and happy recording,
Blueteeth