There is no item currently in runescape called the Spider Cactus. If you are referring to the Potato Cactus, three spawn in the first level of the kalphite lair, in the southwest room inhabited by kalphite soldiers. Seven also spawn in the Kalphite queens lair along the west wall - be aware, gathering Potato Cactii is a potentially dangerous activity, as kalphite soldiers are poisonous, hitting up to six damage, and the kalphite queen is not to be taken lightly.