How do I enforce an outstanding judgement if the plaintiff is out of state?

I loaned an ex-friend $5000 a few years back. I DO have a written and signed contract between us, with him agreeing to pay the money back. This all transpired when he and I lived in the same state, same county(DuPage, IL)- but now he has moved out of state (OK). I sued him due to nonpayment, and won the case, so I have an outstanding judgement in my favor, but cannot get in contact with him. I need to know how to forcefully enforce this judgement- perhaps garnish his wages? I know that is a possibility- but do not know what kind of lawyer (or where to find one in OK) I would need that can help me with that.