Modification of Custody: What to Do when Custodial Parent Can No Longer Care for Children

My question involves a child custody case from the State of: Georgia
My ex was granted physical custody of our two children. I pay child support, garnished from my paycheck, every week. We were never married. Right before Christmas this year she admits that she had been involved with some drug dealings and she was going to go to jail with her boyfriend and she needs us to take the kids. The kids move in with my wife and I right before Christmas and have been with us since. Their mother never went to jail as far as I know. She is living out of her vehicle right now. We had the kids registered within our school district. My ex is still collecting food stamps and child support every week. She refuses to give me the child support card and when I go to the child support office they are absolutely no help at all. They want nothing to do with me since I am not the custodial parent. I cannot afford a lawyer due to taking care of my two kids and still having child support deducted from my paycheck. Does anyone have any ideas in which direction I should go? I can't seem to find any help through the system.

Modification of Custody: What to Do when Custodial Parent Can No Longer Care for Children