In Texas, where do i get a form to modify a child support order?

I'm the non-custodial parent. My daughter lives in arizona, but Texas collects the child support. Custodial parent kicked my daughter out of the house (she's 17 now), and we need to get the state to stop payments to the mother since she's no longer supporting my daughter. We've been told to file a modification to the order. Tx attorney general does not have such a form on their site. Anyone know how I can get one. I can't afford an attorney since child support is $700 per month which leaves me broke in this crappy economy.