Ooni Pro – Pellets:
When you buy the Ooni Pro it comes with the charcoal tray so you can cook using charcoal and wood. Seperately you can buy the gas adapter and the pellet adapter. I am not really sure why you would want to cook with pellets as the charcoal/wood combination is so easy and pellets would be harder but maybe if you run out of charcoal and only have pellets you can still cook pizza!
Ooni Pro Pellet Adapter:
The Ooni Pro Pellet Adapter is available from Ooni for £35 or you can get it from Amazon: