What do I need to do to get custody of my husband's son?

We are having issues with custody of a child. My husband's ex wife has gotten more than 23 total charges against her in the last 4 years. We were getting his son when she would let us. We had some custody papers from domestic relations and I called them and they said they were probably no good. It states that she has primary physical custody of the child but also says my husband has partial physical custody of the aforementioned child at any time. Upon agreement of the plaintiff this order was back in 2003 someone had told you had to redo everything every couple of years or so we kept getting papers from child services of her being turned in for things like drugs abuse and so on. She had a few hearings and got probation for I believe 3 years. The court said that she could transfer to North Carolina but she moved to Texas instead and I'm guessing without telling the court she keeps moving from place to place. Every time we find her she keeps pulling my husband's son in and out of schools just in 2012 she pulled him out probably about 3 or 4 times just to move out of state. We did not know she was even planning on leaving state. I'm worried about my step son's well being with moving so much and living with so many different people. We can't afford a lawyer at this time because we lose about 65 percent of his paycheck for child support. I just cant stand to see her teach him the wrong things in life with the lying, stealing and using people. Thank you.