#1 The Broadway Green Alliance's website (and their Off-Broadway offshoot's HERE)
#2 The on-line guide, The NRDC Theatre Greening Advisor, by the Natural Resources Defense Coalition with the Broadway Green Alliance.
The other best source so far (not on-line) is the book, A Practical Guide to Greener Theatre, by Ellen E. Jones
It's about time! is all I can say.
Ackermann's Repository 1816 - public domain
I'm working on a little greener theater guide of my own, a companion to Alice Through the Proscenium, my how-to theater scenic design book. (Which I'm happy to report is selling steadily. I've seen mentioned from Utah to Kuala Lumpur to Arkansas... pretty cool!) My green tome will have a bit of a different spin on it...