@Mame,
You mean, pro bono cases. I don't believe any are required to take any such cases. If there is a state or county public defender's office, they are probably required to accept whatever is assigned to them, though there would be a need requirement.
If you are speaking of representing the public in civil cases, I'm betting it would be descretionary, and done by some sort of charitible corporation.
To summarize, ask a lawyer.
You're not trying to put legal representation on the same level as medical treatment, are you?