From TheGamer: “Materials are easy to come by in Nier Replicant, but rarely the ones players will want or need for the games trickiest side quests. While a large number of materials can be found out in the wild dropped by Shades or animals others are often locked away to a single location. Vegetables in particular are hard to come by without knowing which stores to shop at or how to take advantage of Niers robust farming system.”
Source: N4G PC Nier Replicant: Where To Find Watermelon