Delamax gray card 3in1 Set (54x85mm, gray card 18%, white, black card, credit card sized, waterproof) (Electronics)
For my white balance I have a gray card, but this is pretty big (close to A4) and unwieldy, so they often stayed at home lying. With this set, I have my gray card and the white and black card always ready to hand. It fits perfectly into any handbag a