You can call the trainers themselves. That's what I did.
Originally Posted by Chrosion
Just look on the Serebii page that lists the trainer rematches, and call those trainers and battle them over and over again.
Platinum FC: 3268 7322 8943
HeartGold FC: 3698 1539 4947
White 2 FC: 3612 1944 2721
3DS FC: 1779-0800-2195
My friend safari: Noibat, Gabite, Druddigon