Gothic 3 Alchemy: Transformation Potions

Where can I find the NPC that can teach me how to brew transformation potions? If there are more than one, who and where are they all? Thanks!

i believe karak the head druid, who is to the far nw of silden is the only one. though some have said porgan the druid who is in the ranger camp on the alternate(long) way to montera from the coastal area. i fairly sure you also have to gain 'thier' trust first too.