I was able to locate a high school friend by contacting her alma mater (college, that is). I found her, though she chose not to make contact with us, her close senior year friends. At least I know she was okay. The point is you might try the library, if you know which system, or her school alumnus society. Probably neither one will give you her address, but you might prevail upon them to send her an email or letter for you, asking that she contact you.
I have luck at times simply Googling the person's name with the state, too.