What you are thinking about doing is genuinely meaningful, and the need is real on a scale that most people on the outside never fully appreciate.
There are over two million people incarcerated across the United States. A significant portion of them have no one contributing to their commissary, no one sending letters, no one accepting their calls. They move through their sentence largely invisible to the outside world, and the absence of outside connection has measurable effects on mental health, institutional behavior, and ultimately reintegration after release.
There is no centralized database that flags inmates by financial need, and inmate financial information is private by law. InmateAid does not maintain a specific list of people in this situation, though if a case comes to our attention we are happy to pass it along.
What does exist are a few pathways worth exploring. Prison ministry organizations and faith-based groups that work inside facilities often have direct knowledge of inmates who have no outside support and can connect volunteers with people who need correspondence or financial help. Organizations like Prison Fellowship, Kairos Prison Ministry, and various state-specific reentry nonprofits work inside facilities regularly and may be able to point you toward someone specific.
Inmate pen pal programs are another avenue. Sites that host inmate profiles often include people who have explicitly stated they have no outside contact and are looking for correspondence. Starting with a letter costs very little and can open into a longer relationship that includes commissary support if that feels right over time.
The impulse you have is one that the system desperately needs more of. Follow it.