Just as a bit of sobering information, there are tons of webpages in the Surface Web and even more in the Deep Web that host CP.

As far as I can tell there's no need to be part of any network of CP creators/viewers in order to access astonishingly large amounts of material. Since OP's ex actually searched normally for these terms, it suggests him finding the Surface Web material which people could just stumble upon without any connections.

There's very little law enforcement can do, short of completely undermining everyone's privacy (which is a whole problem itself). It's basically the same as viewing non peer-to-peer streams of movies in regards to tracking people.