First, was there a forwarding address available? I assume there are no kids because you've lost total track of her. Try the job you knew her to be at last, give it a call, ask to talk to her, and just tell her you're filing to get it over with so you need to figure how to serve her. (I know it will be a bad-feeling blast from the past, but it needs to be done so you BOTH can be free). Or if she doesn't work any more, ask if they know where she went. Or if she has a professional license, the licensing board will have her current professional address on hand.
If you're unlucky with this stuff, then check your state's laws regarding "service of process" this shoudl tell you what the options are. Ask the clerk of court if there is a self-help center available who may give better information that you trying to root it out from a law book.
It's highly likely that you have to hire an investigator to see if you can find her (using similar methods you just did, but maybe they have other options, too... checking the real estate records, the utility records, etc.), and then if they can provide certification that she cant' be found by regular methods (some states won't allow you to be your own witness about this investigation), you can serve her by publication, which is kind of a pain in the neck. There shoudl be some instructions on how to do all this, somewhere at the courthouse.
Or maybe you coudl hire a lawyer for this small part of the process. Just to make certain ti gets done right. Dont' want to spend thousands on an investigator if you coudl have taken care of it with a few hundred to a lawyer & process server. It would certainly save you a ton of in figuring this out on your own.