Judicial boards deal with ethical issues, which admittedly this is. But it is more a criminal matter. In the case of the judges in Pennsylvania, the investigation was done by the F.B.I. and that's usually the case.
But the investigation would be started by who figures out there might be corruption first. Whoever starts the investigation would be very, very quiet about it.
Most cases start by a letter complaining about a judge that fits a pattern of behavior. Local attorneys are probably hearing things, but getting solid information is difficult. It takes someone who can collect from a variety of sources to get enough information to make a case. And then the usual approach is a sting.
The other way these cases come up is when one of the small players get caught, and needs something to trade. Getting a judge is a big bargaining chip in plea bargains.
In the one case where I actually knew the judge (John Fairbanks), the investigation was done by New Hampshire attorney general's office, with assistance from the state police.
Best of luck,
Jim Clark-Dawe