You have the same issue I had. No SSN and no forwarding address. What I found out is that you do not need their ssn to file in small claims court. At least in WA state you do not have to have a process server serve them papers for small claims court it just need to be mailed certified and you can do it at their place of work. Now if you want to garnish their wages my understanding in my state is that I would have to have a process server serve the garnishment papers - certified mail is not good enough if there is no home address. You can also try addressing an empty envelope to the rental once they have moved and write on it service address requested. If they put in a forwarding address the post office will then write it on the envelope and return it to you (make sure your return address is on the envelope). Hopefully you will be able to garnish their wages otherwise you most likely will not see any money.