The Sundrop is a special item that is only found in Dark Cloud 2. Its in-game description reads : "Sap from Himarra monsters. Speeds plant growth." This special item can only be obtained via Himarra monsters, by transforming Monica into a Himarra in order to approach them safely. Go into the Rainbow Butterfly Wood, choose a floor which contains Himarras, like Frightening Forest and once in, transforme Monica into a Himarra when you spot Himarras. Just talk to one of them and you should receive the Sundrop.