What counts as an alchemy ingredient in The Witcher 3
Alchemy ingredients are what fills Geralt's satchel between contracts in The Witcher 3: herbs gathered on the roadside, organs cut from monsters, and the mutagens that come off a decent kill. Each ingredient page lists every potion, oil, bomb and decoction it goes into.
Herbs regrow on the same bushes, so the cheap ones are worth picking on sight. Monster parts come from the creatures in the Witcher 3 bestiary, and the alchemy formulas page shows what each recipe asks for.
Buying Witcher 3 ingredients instead of gathering them
Herbalists in The Witcher 3 restock the common herbs, and buying a stack is usually faster than riding a gathering route. The merchants page lists who carries which ingredient and at what price.
Mutagens are the exception: they only drop from monsters, and the red, green and blue ones feed mutagen slots as well as decoctions, so it is worth keeping a few of each rather than brewing them away.