You can also try AAPCs Project Xtern
https://www.aapc.com/medical-coding-jobs/project-xtern/
I know since the pandemic, some places are not taking onsite volunteers or onsite interns, but you can certainly inquire.
If there are large healthcare systems in your area, they may already have a program.
Smaller independent offices, you would need to contact individually. Explain you are offering your coding certification skills for free in exchange for experience. I would have a cover letter explaining along with your resume. Call and ask for a manager or the manager's email.
I personally recommend if you don't have any medical office experience, rather than work for completely free, to get an entry level position at a medical office or facility that also employs coders. Consider front desk, insurance verification, registrar, etc. Network. Introduce yourself to the coding team. Bring them donuts. Ask questions. See if you can occasionally work with a coder. When a coding position does become available, even if the hiring manager is looking for experience, they may consider you because you:
1) Already know the computer system(s)
2) Already work within the employer, so don't need to go through the hiring process
3) Already interact with the department
4) Already impressed them with your amazing work ethic
Good luck!