Character Pickle Packs can be found on the Recruit screen. Each pack contains a specific set of characters and the chance of drawing them varies when you feed Prismo a Pickle.
Silver and Gold Star characters are rare in the 'Land of Ooo' Pickle Packs, but are more easily obtained in 'Heroes' and 'Misfits' Pickle Packs. To see all of the available characters in a pack, tap the "?" icons.