Need help with legal matter (Perkins)

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My ex has eluded child support for 15 years.
We finally found him and OK state is only making him pay back child support for 3 years.
He has claimed $0 on his tax returns so he has to pay less than $100 a month in child support.
I know he works under the table, he was working as a carpenter when we were together.
His rent is $830 a month so I know he is getting money from somewhere.
I need someone willing to LEGALLY just find out what he is doing for income.
I have to have proof for the state to take action.
This could be pictures of him going to jobsites, going to the bank so I can give the state the name of his bank and they can perform bank levies, etc.
I don't want anything illegal to happen, any contact made with him or anything illegal performed.
I just need a private investigator and don't really know where to start.
If anyone has any suggestions on how to do this legally, without repercussions, I would appreciate it.