This herbalist in Novigrad has most of the common herbs you need. Comment below if you need any more help finding these ingredients. Buy enough to make and it should be enough for you. Buy enough ingredients to make plenty of White Gull. I always use the herbalist in Novigrad, you can find him at the location in the picture below and he stocks both of these in high numbers. If you need to buy some then go to any herbalist or alchemist. By the time I wanted to craft White Gull I had plenty of these items just from looting and gathering. If you cannot view the video then it is the place where you see Priscilla perform her song.
The place in the video below stocks several which is useful. See the video below if you are having problems finding him. Mandrake Oil and Cherry cordial are a pain to find. The others ingredients are plentiful and should not pose a problem to you. It is also a component of Rebis guide for finding the formula for Rebis. White Gull is an important item that is used to craft many high end items. Once you complete his quest then you will automatically learn how to craft White Gull he teaches you. Witcher 3: The Wild Hunt - Mandrake & Cherry Cordial Merchants (White Gull Ingredients)
If for some reason you do not then you can purchase it off the herbalist in White Orchard. Usually by the time you need to craft White Gull then you will already have the formula.
