Thanks all (except GUEST, who seems to take delight in being gratuitously nasty - I consider asking here on the Mudcat as part of DOING research, you asshole). I have a friend who works for the Washington Opera, and will ask her for some leads; Gutbucketeer has also sent me the name of someone to look up.
I think getting a classically trained voice teacher is important, as the training established over centuries of formal music has been adapted and refined to teach effective use of the voice as an instrument, and the mechanics of doing that should be applicable over lots of singing styles.
What I find difficult is figuring out how to pick a teacher from anonymous listings in a phone directory. It's like trying to choose a doctor - how can you find out who's good and who graduated in the bottom half of their class? Just because your best friend's sister likes this particular person doesn't neccesarily mean they are really good at what they do - it may just mean your best friend's sister likes the teacher's personality.
Since there isn't a licensing body for music teachers - anyone can hang out a shingle that says "Voice Lessons" whether or not they have training or teaching ability - I was hoping that folks who have vocal coaches or voice teachers could tell me how they found their teacher. How many teachers did you try before you found someone you could work with? How did you get the name of the person with whom you trained? Did you check up on them in any way, and how?