It's my understanding that he didn't start out as a Pro Bono attny on this case- it just turned into that after the money ran out, then he got approved as her PD but any allowed payment for that is covering other things so he's basicly a Pro Bono now.
In the state where I live, the Bar's magazine advertises for attny's to do Pro Bono work and depending on the type of law (foreclosures/real estate, for example), they assign a willing attny to do Pro Bono work for those in need for a 1 year term. I don't know if they do that for criminal defense cases too, but if I was looking for a Pro Boono attny, I would call the bar in that state and ask for a list of attnys who are willing. I tried finding a Pro bono attny for difficult child and was told there were none for juveniles because they can be appointed a PD if the parent can't afford one. We had no choice in who the PD was- it's who ever the judge assigned and could only be changed if the judge chose to or I somehow proved improper actions by the PD. I have no idea how much difference it would make if difficult child had been an adult or how much difference it makes in a death penalty case.
I dare say that any nationally known case would draw more options for Pro Bono work than the typical defendant and some attnys who thought this would lead to their own fame would be contacting the defendant offering services if they were willing to take it on.