There s a free one from digital tuts but it will not be good for a big scene as you will need to use millions of photons, you can see the light is not crisp in the last scene I posted..................dave

https://cgnotebook.com/wiki/Mental_ra...volume_in_Maya

https://www.digitaltutors.com/tutoria...ustics-in-Maya

https://www.creativecrash.com/tutoria...arti_volume--2