What are the steps to take when trying to get emergency custody of my children who reside in another state?

My ex moved with my kids and is the custodial parent. She had started using again. Recently, she informed me that she had entered a 90 day rehab. She had the kids with her there. She then agreed to sign over temporary custody of the children at least for the duration of her rehabilitation. Before we could get to her, she left the rehab without telling anyone. She now has the kids and I am extremely worried for their wellbeing. I live in another state currently. I am a recovering addict as well but I have already completed a 60 day rehab and I am still currently in classes. I am nearing my 90 days sobriety. I am not sure of the laws which I have to comply with. I need to know the steps that are necessary in preparing myself to get my children. As well as my rights as a father. I do not want to put the children or myself at risk by crossing state lines with them or taking them from her. She is threatening me to never see the kids again and I am not going to stand by and watch her use them as a tool in her cycle.