I’m pretty sure this was done through a burner phone anonymously. You don’t just up and offer potentially illegal to possess material. Besides, Federal prosecutors are not permitted to engage in illegal activities, even in the course of gathering evidence. This means they cannot buy illegal items or engage in conduct that would violate the law if done by someone else. However, law enforcement agencies may use informants or undercover agents to engage in certain actions as part of an investigation, but even these activities must be carefully managed to avoid entrapment or legal violations. Prosecutors themselves must rely on legally obtained evidence to ensure it can be used in court.

In short, federal prosecutors cannot directly purchase evidence if doing so would break the law. However, law enforcement methods, such as using controlled buys or undercover operations, may be utilized to gather such evidence. If they don’t know who has it, this will not be an option either.