Everything from dog fighting rings to child trafficking has dedicated non-profit organizations with paid or volunteer staff doing investigative work. They collect OSINT, do surveillance, and yes, pay informants. As long as you don't break the law in the process (like doing surveillance from private property), you can build evidence to turn over to law enforcement to help open a case.

Something important to note here is informants aren't always in the organization or participating in the activity. It could be a truck stop attendant who texts you when the same guy stops in to buy a case of water and then slips it under the roll up door on his cargo van, or a dishwasher at a cafe who noticed a group of guys with the same tattoo. Things that on their own don't rise to the level of someone calling the police. Payments help people who otherwise would keep their mouth shut because they are in financial situations where they can't afford to lose their job.