If you have their last, first name. You can easily to find wher

I think that you should have their cell phone number that will be kept no matter where moved to.

You can find any one online.  There are many website that provide those information but you need to pay $10~20/month.

Then file a case at District Court.  Also private investigate can find them too (fees).  No matter where to move, you have a case, court can find them.