The primary resource is the Public Defender's Office. If your person cannot afford private legal representation, they have a constitutional right to a court-appointed attorney. They should have been offered this at arraignment, and if they were not or they declined at the time, they can request a public defender through the court handling their case at any point.
Beyond the public defender, there are additional avenues worth knowing about. Law school clinics run by accredited universities often take on prison cases at no cost as part of supervised student training. The quality varies, but some clinics produce excellent work, particularly on post-conviction issues. Search for law school clinics in the state where your person is incarcerated.
Organizations like the ACLU, the Innocence Project, and state-specific legal aid organizations take on certain inmate cases, particularly those involving civil rights violations, wrongful convictions, or systemic issues. They are selective and can only handle a fraction of the cases they receive, but if your loved one's situation fits their criteria, these can be powerful resources.
For ongoing legal needs while incarcerated, the prison's law library is a resource the inmate can use themselves. Federal prisoners also have the right to meaningful access to legal materials, which is federally protected.