Realistically the only acceptable sources are government. For example in the US you basically need a state ID (driver's license), military ID, or passport. And the state systems are now federally regulated (Real ID).
The bills I've seen in the US indicate that you need to use a commercially acceptable solution (so don't mandate government), and include regulations for private providers to follow (e.g. that age validators are also barred from retaining information about the validation).