You can still follow a plant based diet and use supplements derived from non-animal sources.

They're not found in meat either. B12 comes from bacteria out of dirt.

In fact industrial animal agriculture frequently has to supplement b12 to animals as they are commonly deficient.

This is just not true. You're probably thinking of vegans who aren't allowed to have cheese, milk, and eggs. Vegetarians can easily get B12 in their diets.

Vegetarians get B12 from the animal products created from animals which supplemented B12. Everyone is supplementing B12 regardless of their diet, it's just a matter of how many steps removed from the supplement that people seem to think makes a difference (it doesn't).