You would probably be best off trying to use some type of attorney, if you can find a legal aid department in your state that would be great, they are free
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You can call your local court and actually try to ask how to start something on your own, they will give you the forms you need to try to take him to court and can even provide you with numbers to attorneys near you.
Most good services to find someone are ones you have to pay for, and trust me if you get an attorney they have programs they use that can tell you how many freckles are currently on your dad's face
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. Well okay, it's not that great but it's pretty good, it's freaky to be honest how much "big brother" knows about us.
If there was a court order that said he had to pay this amount of money and he's not sticking to it your mom should complain to the court that issued it let the judge know, it's possible that the system will search for your dad and take him into contempt until he does cough up some payments. I'm not sure how exactly you'd go about placing the complaint but once again, your local court can probably head you in the right direction.
Good luck with your search.
EDIT:
If your locations is accurate here on ppt, I think this is your states legal aid offices
http://www.mobar.org/member/legalaid.htm
You can always try contacting them and seeing if they can help. Legail aid offices are usally offices dedicated to providing legal services for low or no money at all.