I'm assuming you're using an English Firered rom.
Trainer Card Tileset: 0xE991F8
Trainer Card Palette: 0xE99198
Trainer Card Tilemap: 0x3CC6F0 (not 100% sure on this one)
That's all I'm going to say. You can figure out the rest.
EDIT: Actually, I'll be nice. Get your new Trainer Card background and palette, and insert them using unlz.gba. Don't check "Automatically fix pointers". Take note of the offsets you inserted the background and palette in.
Next, find the pointers for the Trainer Card tileset and palette, (F8 91 E9 08 and 98 91 E9 08 respectively) and replace those pointers with your new pointers. Repeat process for tilemap. Voila, new Trainer Card.
EDIT 2: Whoops, forgot about the tilemaps. I'll update you on that later.