Unfortunately, Custap berry can only be obtained through special events.
Here are the events:
>Given to players who ranked in the Christmas Cup Wi-Fi tournament for Japanese Black 2 and White 2 or the 2012 VGCS Winter Battle Wi-Fi tournament for international versions. These took place between December 21, 2012 - December 24, 2012, and December 13, 2012 - December 17, 2012 respectively.
Hope I helped!