A cackleberry is a slang term for an egg, from the resemblance of an egg to a fruit, and the sound of the hen which laid it.
Go to the Brightvale Fruits, and select the cackleberry, and they give it to you for free.
in brightvile look for a cackleberry
If your looking for the first fruit on the negg festival quest go to brightvile fruits and look for a cackleberry if it's not there then just go to the shop wizard.