People...it's called the
Mystery Part; not the Enigma Part. You can get each of the items, Stone Plate and the Mystery Part, from optional missions. There are also Wondermail codes to get the missions for the 2 items.
May make random appearances every-so-often. Trying to officially return...
POKEMON BLACK FRIEND CODE: 1249-2602-6566
POKEMON BLACK2 FRIEND CODE: 1679-1583-9820
POKEMON Y FRIEND CODE: 3308-5051-2299