You don't think?
1) You must have gotten one a long time ago, but you just don't remember. You either
• Found it on a berry tree
• Received it from an NPC; of the like.
2) You actually did trade to get one, or got one of these event Pokemon, which both have a 100% chance of holding a jaboca berry.
• Tanabata '08 Celebi
• 13th Movie Jirachi