For plant extract prices and alchemical ingredients, see here.
Plants in the game grow (mostly) in areas suited to their needs. Therefore, not all plants can be found in all locations or even in every Chapter of the game. White myrtle, celandine and sewants are the most widely available. Some players might argue that wolf's aloe does not grow wild (except in The Price of Neutrality), but it does replenish itself despite being potted.
Most of these herbs can be purchased, except ergot seeds and hop umbels as these do not actually appear in the game at all. Mandrake is also received as a gift on two separate occasions.