i had this discussion with my lbs. looking for a stiff insole for my vans as well. seems that they make them, but they are specially built for the more narrow riding shoes.

obviously my first instinct was narrow = ok, it will still fit in the sneaker right? thats true, but apparently there are issues with the insole sliding around and becoming more of a hindrance than assistance.

im going to keep my eye on this thread tho, as i imagine this is something that many fg/ss riders are lusting after. good question....