How do I get a divorce when I don't know where my spouse is?

In a case like this, it's usually possible to "serve" the defendant by publishing a notice in the local newspaper, with the court's permission, and this is considered "notice to the world."  This is what's called a legal fiction, especially if it's a small-town paper, but you can use that fiction to your advantage.

If you still have a record of his social security number, that might help you to track him down -- or to find out that he's no longer among the living, in which case you wouldn't need a divorce at all.  A good lawyer can help you with all of this.

You can get a "divorce by default".  You will need to have your papers prepared and file them with the court.  After filing, a Citation will be issued and your spouse served.  In this case, you can "serve" a divorce complaint via "notice by publication".  This is used when a respondent-spouse cannot be located by the petitioner-spouse.  Basically, the respondent is notified of the divorce proceedings by publishing such notice in a local paper in the area of their last known whereabouts.  Then, if  after 20 days your spouse does not answer (ie they default) then you will file your final paperwork and appear before the judge on or after the 61st day.  A divorce will then be granted on the terms that you requested.