Nozomi Supplement?

For future trip, I would like to know how to purchase the Nozomi Supplement to ride from Osaka to Tokyo. Does this purchase have to be in person or can it be done at a kiosk? I saw the guide said to purchase a special ticket for it? Exactly HOW please?

I’m asking because on July 18, my son and his friend used the kiosk in Osaka and they didn’t know they picked a Nozomi train so they had to pay $97 US for that trip to Tokyo.. They didn’t know about buying supplement. Their 7-day rail pass was not worth it at all.