how do i get a divorce if i dont know where my husband lives?

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 it in a local paper in the area of their last known whereabouts. 

Then, depending on what state you are in, they have to reply within a certain amount of time.  If they do not, you can proceed with a default divorce.  Each state has rules about how you go about filing for a default divorce, but it can be done.