In a pinch, I've balanced off t-shirts, walls, sheets of copier paper... but none of those things are necessarily "real" white... just look at the differences of "white" paint chips. A gray card is cheap and easy to store in a camera bag - the ones that don't come with cases usually fit in a plastic DVD case. (I have purposefully balanced to a "ultra-white" paint sample, since those usually have a lot of blue in them - gives you a little warmth. Many fabrics and paints billed as "super" or "ultra" white are really pretty blue...)