How can you find someone's address?

My 6 year old son was stolen by his father who does not have custody, and is hiding the child. He is not letting me know his address, which is a violation of the divorce decree. The police won't help us because it's a civil matter and would have to go through court, but we want to get him asap so he can attend his kindergarten graduation.so how can I find where his ****** lives? All I know is he lives in Houston, and I know his full name and SS number. I also know the full name of his girlfriend who lives with him. Any ideas would be greatly appreciated. I worked all day and night since 2 Saturdays ago trying to find him and I'm at my wits end