An attorney should be able to help you. I know IL and many other states have procedures in place to deal with an ex no one can find. I know in IL, my sister had to place ads in major papers for so many weeks/months notifying "ex" of the court date regarding "XX" children. If he is a no show the judge can proceed from there. Even if parental rights are terminated, ex will still be required to pay for any arrears up til the dates of parental termination unless the ex shows up and asks to settle for less (my sister's ex did; he would sign his rights if his arrears were forgiven). Now in his case, if the judge was mad enough and chose to he could have accepted the parental termination rights and still assigned arrears to be paid. This judge went with allowing the forgiveness so my siser could cut him out of his life totally.
If you can't afford an attorney you could speak to some if they have no/cheap consulation fees or go to a local law schools family law clinic or see if you qualify for low income legal help (that is what my sister did).
Thanks,
awakenlynn