I've never actually used mixi and I'm about to ditch facebook lol. I usually just to stick to Cure, but that doesn't really facilitate groups and such.
CelestialShadow might know of some though. She lives in Tokyo and makes all of her costumes 100% from scratch. She does an amazing job and takes commissions too. She'd know the best fabric and supply stores in the area.
For armor and heavy duty props get with Goldy. He holds classes at a workshop in Akihabara. Goldy is one of the top armor/mech cosplayers in the country. Couldn't ask for a better teacher.
Summer Comiket 90: Tokyo, Japan
Military Odyssey: Detling, England