the inmate locator for the prison is the easiest way. if they are not in state or federal try vinelink.com
https://www.inmateaid.com/ask-the-inmate/how-is-the-easiest-way-to-get-an-inmate-number-for-an-inmate-so-i-can-send-a-letter#answer
Accepted Answer Date Created: March 04,2021