How to go about receiving child custody.?

i live in arkansas.my ex and my kids live in california.my ex-girlfriend (of 10 years) lived in albuquerque and was frequently taking my kids across the mexico border to visit her wanted felon boyfriend without my permission..the kids told me in detail of the many times i happened.she denied it.the u.s. marshalls know of her relationship with said felon.she then disappeared for about 7 months with my kids.her family in albuquerque didnt not know of her where abouts.she stopped taking the kids to school and abandoned her apt.i filed a missing persons report for my kids. she called me months later from mexico from a private number.she told me that she lived in juarez with the kids.she didnt want to explain why.just that it was drug related.she then moved deeper into mexico for no apparent reason.the kids did went to a spanish only school.my kids hardly speak spanish.she began telling me she wanted to leave mexico but she was afraid the police were after her.PARANOID.she told me if i can pick her up at the border so we can attempt to fix our relationship.i agreed.we came back to arkansas where she spent a jobless month before leaving to l.a. to live with her parents.the kids where enrolled in school here but that didnt stop her from leaving because of "depression".she continues to be jobless living out of her parents living room with 3 kids.she now is planning to move again.i want to put a stop to this once and for all.i have a steady job, for a few years now.i actually live in a nice house.i have no criminal background nor do i do drugs.i did attempt suicide 3 years ago by slitting my wrist.that was the only breakdown i ever had in my life.im not sure how to go about starting a custody case or proving paternity.also what are my chances of actually having the kids put under my custody and are there any pro bono custody attorneys? im not wealthy at all.but i can provide for them.i love my kids and i hate to see their lives ruined by an unfit mother.