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All around you. Buy all the alchemy books you can and level up the appropriate bronze-level skill so that you are able to harvest herbs. In Act I, Abigail sells a couple of books, as do one or two of the merchants outside the Inn. Just examine each ingredient you come across to see what alchemical component it contains.

you can buy ingredients from herbage vendor in Wyzima city
btw how can I kill the plant with red head on the swamps ?
it keeps shooting at me and when I try to attack it Geralt can't come closer to it and can't attack so how to fight with it ?

That isn't really related to this thread, but you have to dodge (double left click in mouse mode, double press direction key in WASD mode) to get in close to use the sword (silver - fast style). You're better off hitting them with the Igni (fire) Sign though - much quicker and easier, especially when you are facing multiples.

as has been said before early on in act 1 you can get both the knowledge and ingredients for the cat potion from abigale, and to collect the ingredients yourself you need the knowledge to harvest them from the plants lying around, no point in using the torch as i have found using the cat potion works best and lasts longer and also frees up a weapon slot.